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91 C2 Hollow Ringing Sound in Engine Bay

 
Old 03-15-2004, 12:47 AM
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chobe22
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Default 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.
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Old 03-15-2004, 03:07 AM
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John Boggiano
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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....
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Old 03-15-2004, 03:30 AM
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Bill Wagner
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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.

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Old 03-15-2004, 03:32 AM
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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!
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Old 03-15-2004, 03:54 AM
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Adrian
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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?
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Old 03-15-2004, 04:16 AM
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John Boggiano
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Originally posted by Adrian

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.
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Old 03-15-2004, 04:32 AM
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Mr. C4
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Check the fan belt tension sensor, common culprit for "ringing" noises.

Ciao,

Johannes E.
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Old 03-15-2004, 08:24 AM
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Fred, Long Island
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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.
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Old 03-15-2004, 12:49 PM
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joey bagadonuts
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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.
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Old 03-15-2004, 03:10 PM
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John Boggiano
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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...
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Old 03-15-2004, 04:10 PM
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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?
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Old 03-15-2004, 04:32 PM
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John Boggiano
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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?!
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Old 03-15-2004, 04:54 PM
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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??)
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Old 01-02-2006, 12:26 AM
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Default 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
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Old 01-02-2006, 01:17 AM
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MarkD
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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.
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