91 C2 Hollow Ringing Sound in Engine Bay
 

Hi Everybody,

I recently picked up my first 964 (91 C2 Coupe 48k miles) from out of state. It wasn't until after buying it that I discovered Rennlist, Adrian's book, and a host of other key sources of information that I should have known about *before* I bought the car.

From the sounds of the 964's reputation, I think I dodged a bullet...the car had no service records until my own, but I did have enough sense to have one PPI and one post-PI by two different Porsche shops done...both of which, after their comprehensive inspections, proclaimed the car to be one of the nicer 964's they've tested. Since then, I've had the valves adjusted, spark plugs replaced, distributor vent kit installed, fluids replaced, and a host of other reconditioning items performed.

Everything feels strong and happy, except for one issue that I have yet to tackle. So, I thought that I'd introduce myself, and see if anybody has had a similar experience with their car....

My engine makes what I can best describe as a fairly faint, sometimes not-so-faint, ringing sound above the music of an otherwise strong motor.

The noise is a sort of hollow ringing sound, as if a relatively light weight metal ring 4-6" in diameter were vibrating within a thin walled pipe.

My mechanic thought that it might be one of the engine's belts nearing its service life of all things, but he didn't seem supremely confident of that diagnosis.

The sound is most noticeable right after start-up when the engine is cold. It changes with RPM, but not necessarily directly so. It either dissappears once under way, or is drowned out by engine and wind noise...I can't tell. When idling at a traffic light, the sound is less pronounced to even non-existent (I'm assuming due to a warm engine). At these times, it drives me slightly batty, in the same way as when you think you hear your mobile phone ringing and discover that sometimes it's not.

Before I go down a potentially expensive quest for the elusive sound, does this sound familiar to any of you? Does my description sound like it actually could be one of the belts? What would you recommend as a troubleshooting strategy, maybe starting with a technique for localizing the sound more concretely?

Thanks for any insight that you can provide.:)

Welcome aboard!

Sounds very familiar to me - I've been wondering about it myself!

My car has done this for as long as I can remember (we're talking many years) and it is slowly getting louder.

I think it is probably the fan/alternator bearings getting somewhat worn. I've actually been wondering whether to change them before much of this year's coming activity, so I'll be interested to see what others think on this...

P.S. I like your description of what it sounds like... my wife always comes out with these descriptions for strange noises - things like, 'It sounds like two small coins trapped in a metal pipe that's open at one end and has some water in it. Or maybe oil. And someone is spinning the whole thing around gently.' Etc....

I hate to state the obvious, but it sounds like something is loose....like exhaust mounts, or for that matter anything else that may have come loose with age.

:cheers:

I think you were looking at the end of my post, Bill. That bit was about a metaphorical noise. I just put that in to confuse! ;)

Sounds like the engine driven fan problem to me. Could be low on tension due to an aged belt or incorrect shimming of the pulleys which provide the belt tenison.
The fan shroud may be loose or the bearing is on the way out.
Ciao,
Adrian
964C4

PS: Could this be the whine that Peter Morgan refers to?

I've wondered that, too, but it really doesn't sound like something that would be described as a whine.

The problem reminds me, 'though... a couple of years ago, after a trip to the Nurburgring, there was some evidence of contact between fan blades and shroud. There was some paint (red painted fan) deposited inside the shroud. Guess I'd better get those bearings changed.

Check the fan belt tension sensor, common culprit for "ringing" noises.

Ciao,

Johannes E.

Fan pulley halves. If the rectangular hole in the center of the half wears, the pulley moves laterally and creates the ringing sound. Had it happen to my old SC. Not enough shims in the shaft caused the gap, allowing it to wear gradually until it rang.

I used to have this problem--or one that sounds very similar--and it was due to loose shielding around the catalytic converter. After replacing my cat with a bypass pipe, the sound disappeared. It was during this removal/installation that I discovered the loose shielding.

If you remove the left rear wheel, you'll see a two-piece shield which wraps around the cat where it meets the Factory header. The pieces are held in place by small bolts which can come loose or fall out. Tightening everything should eliminate the rattle.

It's not a rattle, Joey.

That was just something I made up to illustrate how Paula talks about these noises. It's a ringing.

This is going to make a good Monty Python sketch if there's ever a comeback...

Yes, the sound I experienced was a faint ringing--more pronounced on start up--just as chobe22 described. Do you still have your cat, John? Does anyone without a cat hear this sound?

Yes, I still have my cat, but the noise doesn't sound like there is something loose. Actually, rereading Chobe22's post has me thinking mine is something different from his...

Mine is quite like the noise you get from running a damp finger around the rim of a glass...

P.S. Mind you, tonight Paula has announced that it is like a shoelace (yes, really, that's what she says...) with a pair of metal disks on it, so maybe it is the same after all! Even I'm getting confused now!

Shoelace?! The car has one of the world's greatest engines, the most glorious piece of aural delight and she says it sounds like a shoelace?!

My Lord!!

I wrote that post less than 24 hours ago, got so many great leads to follow, from around the world, with an answer from the guy who wrote the book I'm studying, and a case of unstoppable laughter from John's description of his wife.

I am IMPRESSED.

Many thanks, folks, for the feedback. I'll let you know what happens.

Best,

Patrick Stanton
1991 C2 Coupe
2001 Honda Odyssey (can you blame me for getting the Porsche??)

Ringing noise
 

Hi all,
I should let you know I have been reading Rennlist and other sites for approx 2 years learning about aircooled Porsches (I had a 951 for 3 years) while at the same time saving my money for a 964. I recently purchased a 91 C2 with 69K in Tampa and drove it back to Texas. 1600 miles in 2 days. The car is in great shape and Im thrilled to own a Porsche again, it runs great. I had a PPI in Tampa and they gave it a clean bill of health. Ive since found a few things wrong that the PPI missed but nothing I cant fix, I hope.

I do have the ringing noise John describes in his earlier post. After reading it could probably be the fan, bearing, shims, tensioner, etc. I immediately went out and started the car and listened with a steathascope to the fan housing as well as the rest of the engine. I still cannot locate the source for the noise. I can see some scrape marks on the inside bottom of the fan housing when shining a droplight through the fan while its running. There seems to be a pretty tight clearance between the fan and the housing all the way around.

John Boggiano or Chobe 22 did you fix your ringing noise? If you did how did you do it. Thanks in advance! Im a paid up member of Rennlist but I just joined and it hasnt showed up yet.
Craig

Craig,

If you have scrape marks on the inside of the fan housing, it is likely the alternator bearing is going out. It is a really good idea to replace it asap before it makes MORE contact with the shroud. That would be.. bad...
Another quick check is to remove the AC and alternator belts, leaving on the fan belt, and spin the alternator shaft. If you see the fan wobble within the shroud (it will be obvious) the rear bearing in the alternator is going out.

It could also be the front bearing the the fan rides on but I hear that is less common, though almost as costly as the alternator.

Ringing noise
 

Mark,
Thanks for the quick reply, I have tommorrow off, so this will give me something to do.
Thanks
Craig

This happened when I replaced a part of my exhaust. It was the exhaust mount on the bypass pipe in the very rear. When away once I moved it a round a little and tightened it up.

CraigC,

When are we going to see pictures (in a new thread) of your "new to you" 964???

:)

Hi Chobe22,

Welcome !

I have the same problem and I tend to agree with Adrian, it is in my opinion, definitely the fan bearing!

E Chan

I took the belts and pulleys off and the car still makes the ringing noise when running although it doesnt seem as loud. The shaft spun smoothly and both bearings feel smooth when rotated by hand. The fan does not contact the fan shroud when spun by hand or after I put it back together and started it up. I painted the fan shroud black inside where it appeared there had been contact in the past. If it touches again it will scrape the paint off and I will know it has just happened. I can move the shaft a zillenth of an inch up and down, is that to much? is there supposed to be zero movement? I think there is a good chance the ringing may be in the exhaust, or possibly I have the fan-alternator ringing and the exhaust also ringing. I looked through the records that came with the car and it appears a shop reinstalled the fan shroud at some time but it makes no mention of alternator or fan bearing replacement.

Im glad I did this, as it gave me a chance to check out my tool kit that came with the car. None of the tools I needed to change the belt were in it, and if they had been I wouldnt have been able to change a belt by the side of the road because the same guy put on the fan nut that installed my transmission drain & fill bolts! It must have been torqued to about 300 ft lbs.
Any comments on the acceptable amount of play in the shaft is welcome.

Terry, Your avitar is my new to me 964 and your old car. I will try to post some pics although you might have better luck posting pics than me.
Thanks,
Craig

old thread new bump -

I have this same noise - I am replacing my alternator this week - but I don't think my noise is the alternator - I think its more coming from the exhaust - but it's definately wierd - and seems to be worse if I am cruising slowly at 2500 rpm on a downtown street, sounds like a couple steel balls in a can....hollow knocking type thing....anyone familiar with this noise?

My first guess would be cat heat shield.

I replaced my alternator yesterday - and MOST of the hollow clanging sound is gone - but some still remains..so maybe it was a few things together making the noise -the alternator bearing was definitely toast....so now not only do I have that bearing out of the way - the alternator was only making a portion of the power it was supposed to put out - and now the car is a rocket...which is nice....

on to the install of my cup pipe, and to look into clanging heats shields and such - and maybe come across the root of the other noise...

Resurrecting an old thread. This hollow marbles in a tin can noise is still present. A lot of other guys have experienced this. It's not the alternator. It might be heat shields, but when the motor is idling, you'd think the noise would be worse due to more vibration, and it's less noise, not more...

It's killing me. I get these ideas in my head like rod knock, but then the motor sounds fine and the noise is not present in the motor, it seems...so can't be. And then I think, maybe it's the planetary gear...and God only knows. Then maybe I think could it be the flaps inside the heater blowers that blow heat behind the wheels????? Is that possible?

Has anyone experienced this noise and fixed it?

Have you tried locating the sound with a ? The 12-18" or so hollow tube on the end really helps to pin point the source and they only cost $15.

91 C2 Hollow Ringing Sound in Engine Bay
 

A broomstick (or other short wood dowel of similar size) held to your ear / touching various parts of the engine also works in a pinch if you don't have a mechanics stethoscope.

Marbles in a can is a typical description of a worn chain, guide or collapsed tensioner.

As unlikely as it sounds I've had this thread open in my phone for a long time. Initially I thought something really bad is happening, until recently I met another new 964 owner, who also had rebuild, and when he started his car the same ringing was there. Based on previous posts on here I thought it would be the fan bearing, but now I'm not so sure.

Its loudest at start up and fades a bit once the car warms up, but even then its very faint and can be heard if driving by barrier so the sound shoots off it.

I have some semi-frequent chain drag/rattle on start up, but I've heard this is on the normal side until tensioners pick up the slack.

I thought maybe those as well...but they were all replaced a few weeks ago. So no dice there...I wish one of the many things I had to do anyway would have resolved this.

Oh and here is another tidbit. The frequency of the marbles in the can doesn't change with the rpm...but it does go away and return it seems. This is why I thought maybe chain guides. But now all those are new. So I'm lost.

Who else has the sound?

Any chance you can get it on vid?

I'm 1000 miles away from my car this week, so can't today, but yeah good idea. Not sure how I can do that, since it doesn't happen with the car idling. But maybe I can borrow a friend to stand on the road and see if he can pick up the sound on a drive-by...

loose parts inside muffler or Cat?

Nope. Have had both g pipe and cup pipes on , and the cup pipe used a different cat. Also replaced my heat exchangers last year. So each component has been off. Never affected the sound.

+1

I hope not, I'm already replaced two!

This sounds like a good possibility...

+1

But then this thread seems to change (from a faint ringing noise, now the focus is a rattling noise... Funny, I hear both noises coming from my car.

I hear the same rattling noise, seems as though it's coming from the passenger side. I hear it at low speeds and it sounds like it happens after going over a small bump in the road, I thought it was loose bumper hardware shaking (rattling) back and forth after a bump. Everything I've touched seems solid, but I need to get it up on a lift...

No cat, and it sounds like it's coming from the other side.

I only hear the "hollow marble noise" when driving down a bumpy street at low speeds. Not while idling.

+1 frequency of the marbles in the can doesn't change with the rpm.

I don't have a video (yet) of the marbles, but I found a video of the faint singing noise.
I heard it could be coming from the harmonic balancer? (I can see a slight wobble)

Took a little while to capture the sound, it seemed that on the days I had a camera with a decent microphone ready the sound wouldn't be as audible. So here is what I have from three different days. Might have to turn up your volume a little.

Starts being distinctly audible around 15sec mark

Also, excuse the butt to the camera in the beginning, I had just replaced fuel filter and was checking for leaks.

Alex,
I think I can here it in your vid. It's faint though.
Do you hear the same noise in mine above??

It is tough to get a mic to pick it up well, but it is a faint sound for the most part in my case. Loudest at start up. I don't hear it in your video, to me it your sound is more a steady whine than a ringing.

Ah....

Now you guys know you bought air cooled cars right? :thumbsup:

I thought you had a harmonic banging or grinding, sounds normal to me. These things make those noises. Unlike a water cooled motor there is nothing to dampen all the mechanical internals. Just tap on the fan, shroud, cyl cooling fin, even a lwfw all make a particular ringing sound, the fan makes a howl that I've found particular to each car. I know you know all this but my point is these motors just make these noises, it's part of the charm, enjoy the drive:thumbup:

Thanks Alex and J.

Like you said, these cars all make different noises (and each drive differently too). I can hear the ringing, tinny noise... but I guess it's all part of the cacophony of noise coming from back there.

With that settled, I'll start researching more about fuel lines... lol.

To the other noise I was hearing....

I think this is the cause of the rattling noise I hear in town when I go over bumps in the road:

91 C2 Hollow Ringing Sound in Engine Bay
 

I realize these cars make noise. I can't tell Alex if yours is like mine. So next NE9O meet we can have a listen...

My noise, and the noise that has been described, is not "noise these cars make". I've heard plenty that don't make the same sound.

Still hoping it's something easy or benign...so far the motor hasn't fallen out the back...knock on wood...

Mine does it too.

I think it's exhaust related. Something just needs tightening up.

'94 C4

in the clip above does the audible ringing sound go away when you close the door?

Have you checked to ensure the fan isn't contacting the housing at any point?

I do agree that these cars can make a variety of sounds, and smells in some cases, but I don't really the sound being there for the first two years I had the car. But as a new development, if I drive next to a wall or barrier on highway, I can hear a slight raspy tinkering, and in some cases it will increase with RPM. So I'm hoping its something loose, just can't wrap my head around what it could be. All exhaust clamps and mounts are tight.

Guys,

My point is not that you don't have "atypical" noises, is that they all have atypical noises. The more subtle and harmonic ones are part of the mechanical nature of the beast. You will drive yourself crazy trying to run them all down on a 911. Check everything over throw it on the rack and look for loose parts and broken bits and if oil pressure and leaks are good just drive the car...

I'm convinced that the engine under trays on these cars was as much about dealing with the rattling nature of the drivetrain and "seepage" as it was any sound restrictions or aero... These simply will not be anticepticly quiet like most modern cars.:icon107:

91 C2 Hollow Ringing Sound in Engine Bay
 

I agree w you...and appreciate the comment...so don't get me wrong.

That said, I've put 30k on my rebuilt motor with this noise present the entire time. And it's my daily driver, so I'll keep driving it hard till it breaks again, and then it'll get fixed...I'll then still likely have this noise. It's summer, windows come down, garage isn't freezing cold, so this is my shot to fix everything before winter comes again.

A few weeks ago, I put my g pipe back on. And same as last summer, it drowns out the noise. I'm now less concerned. Lol. It's there though, quietly lurking underneath...

I had the hollow knocking noise which most thread point towards:
the fan housing/ fan due to:
1. alternator bearings
2. belt tentioner
3. broken distributor belt
as possible causes

Having just cleaned up and reviewed my OEM tool kit I knew where to find the two tools needed to remove the fan belts and I think I found the source of the noise - the main pulley has 3 allen head bolts in it and all three were less than finger tight ie loose - the metal they're made of feels quite soft but I tightened them up with a rachet allen head socket and tested engine for 5 min and the hollow sound of the fan hitting the housing was gone.

Should I use some removable blue 242 loctite on them?
Anyone else found those 3 bolts loose before?

cheers
Matt

You should definitely use locktite on those bolts. I think the torque spec is 7 ft/lbs? I torque. Spin the motor, check torque, spin the motor, check torque again. So I know the belt tension hasn't affected my initial torque...

Thanks GG, will do just that thanks.

(Ps - watching your group chassis refurb with jealous ... nothing like repeating these tasks a few times in short order to become very proficient and make lots of happy guys ;))

Just to check - do I spin the engine a few times before all the secondary pulley & belt is back on to just test those 3 allen heads ... or do you mean once both belts are back on?

I have had this noise for quite a while but today I managed to find the culprit. It was caused by the belt sensor arm sometimes slightly coming into contact with the outer fan housing. As the small wheel rotates it sends a resonance through the plastic arm into the fan housing giving that distinctive faint ringing noise. Loosening the fan strap and reseating the housing gave just the fraction of clearance needed to stop the plastic arm coming into contact with the fan housing et voila...no more ringing!

Thanks for "chiming" in ras62! My 964 has that ring to it too (motor recently rebuilt). I'll check the belt sensor arm.

Thanks for this. I've had this for years. It's sort of like a chime sound. Sort of pleasant actually, but I'd love to know it's nothing too serious.

Can you snap a pic of the area I should check?

Thanks!

Wow. For five years I've been trying to track that one down. Makes perfect sense...

It sounds like quite a common issue if it can be called an issue. Mine has also been ringing away gently for a few years LOL.
Tmistry, look at the lower left corner of the cooling fan housing where the belt sensor wheel touches the belt. There should be the tiniest gap between the sensor arm and the fan housing. If there isn't a gap then the fan/alternator unit is sat too far back.

+1

Very interesting, been meaning to update this thread as well, but my ringing went away after swapping out the oem exhaust+headers. I assumed it could be the flapper boxes just lightly moving back and forth.

My 1989 C2 (289K kilometres) is ringing for years now ! I would get worried if it went away ��.....as long as she goes like a tiger with good oilpress and temp, I am happy !!!!

p.s. Just received a ticket for speeding.. I am not happy now, Holland has more speed camera's than a stray dog has flees .....d*mmit

ringing noise revival
 

Listen for the ringing noise in the video. Using a stethoscope, its seems to localize to the exhaust system so I'm hoping it's the cat shield or some other loose nut or bolt external to the motor. The ringing fades as the car warms up. Will report back once identified. '90, 105k C4 with stock exhaust.