EJZero1

A while back I noticed the onset of the occasional vibrating sound coming from the 944. Took it to a european auto "specialist" who gave it a going over and proclaimed my autotensioner roller was going bad, and he would charge me only $1400 to change it out for me...

Instead, I took it as an opportunity to learn something new. Bought the Arnnworx tool kit and changed it out myself today.

Good News: The job took forever, but went well. No timing issues. Bruce's tools worked great for this first timer. Put her all back together, crossed my fingers, held my breath, and turned the key. Started up great and runs like a champ.

Bad News: The sound persists!!! Took her out for a ride, and here's what I've been able to deduce. Happens at 3500 revs and 3500 revs only (Ok, +/- 100). In motion or parked, rattles at 3500. A very "tinny" and "rattly" sound, a bit similar to what a grinder on sheet metal sounds like. If you're accelerating to shift, it sounds for that 3500 +/- window and then it's gone. Decelerating, same story. If I park and hold it at 3500, it will rattle continuously. It is not a gradual onset that starts and gradually increases or decreases in various rev areas. Like I said, present in that ~200 RPM window only (3400-3600).

Had my wife climb in and hold her at 3500 while I snooped around. Popped the hood to listen and found I couldn't verify it comes from under the hood. Standing next to the car, it's loud and clear, but can't zero in on it in the front or back of the car.

I know, vague as all get out, but any ideas? Rattling hardware somewhere?

Maybe tomorrow if I have a minute or two I can do a quick movie for your listening pleasure...

Thanks in advance for any considerations.

tblofsky

first things first. you need to determine where the general area it is coming from. When you rev it up to 3500 rpm's is it in gear with clutch in or is it in neutral with clutch out? That will determine weather it is engine related or clutch or torque tube. After that then you need to take a wooden dowel and stick one end up to the engine and the other end up to your ear, Have your wife rev the car and you listen closely. By moving to diff. spots on the engine you will be able to tell if it is internal to the engine. If that yields nothing then you need to feel over everything and see if anything is loose. Could be just a metal rattle from a loose bolt. Hope this helps

tblofsky

Also did you replace the Timing belt and the Balance belt while you were in there?

Quikzilver

Check your cat converter heat shield. I've had cars that have made the same type of noise.

V2Rocket

balance shafts out of time.

when you floor it in 1st or 2nd about 5000rpm does the car sound louder too?

EJZero1

Thanks for the responses, everyone.

Car came with a receipt in the glove box for water pump, belts, seals and much more just about 5k miles before I bought it, and I've only put about 3k on it since. Belts still looked new when I was in there yesterday.

Heard about cat heat shields producing similar noises, and will check it out when I have some time to climb underneath.

As far as the balance shafts, the sound is only present in the aforementioned rpm window. Never hear it at any other engine speed, but not averse to checking it out. How would one tell if I have balance shafts out of whack?

For what it's worth, took her out for a drive today and symptoms are unchanged. Runs and pulls strong, with the only hint of any issue being what I've already described. Will be snooping around underneath looking for loose heat shields or any other obviously loose hardware as soon as I get a chance.

Thanks again for the inputs!

Grandpa#3

Just don't forget to re-tension the cam belt at 1500 miles. Metallic sounds can drive one a little crazy until they are found and tightened up. Bruce's tools are the best around. I bought them and the rear wheel bearing tool. They have all ready well paid for themselves.

Cheers,
Larry

StoogeMoe

Sounds to me like the person who worked on the car before you bought it, has the balance shafts on wrong. That the tell-tale sign with a vibration at 3500 rpm. Double check the alignment marks and the O and the U on the balance shaft gears.

This vibration is causing a loose heat shield to rattle.

Barry A. Waters

Since this still seems to be a 'WTF?' type issue, I'd expand your search to include checking your motor mounts, a common issue w/these cars. I had the same 'mystery tinny-rattle' and it progressively got worse until the rear muffler added a 'thump-thump-thump' when taking off in first gear.

When a motor mount collapses, it 'leans' the engine to one side just enough to throw the exhaust system's alignment out and, since it vibrates anyway, that will produce all kinds of neat 'rattles' if the exhaust system comes into contact with anything along its routing (and there are PLENTY of things to contact).

New mounts made my '88 944 NA like a new car! I actually looked at them when I was initially attempting to diagnose the problem and expected to see evidence of 'leaked fluid' if they were bad but that wasn't the case - nice and dry but still collapsed as I later found out!

It's a measurement spec thing (not necessarily evidence of a fluid leak) that you want to use to examine the mounts. Let me know if you cant' find the specs - I think I can locate them if you need them.

Best of Luck!

Barry

EJZero1

Cracked me up...

Crawled under yesterday to check the heat shield. Appeared to be evidence of consistent contact between the shield and underside of the car. Made some slight adjustments which had some effect on the sound, but not totally gone. Next move is to just remove the heat shield entirely to see if it eliminates the noise. If so, great! If not, at least it narrows the search.

Will do some homwork on motor mount specs and then check them. What's the degree of *** pain involved in changing out bad mounts?

Balance shafts not ruled out yet either. My only comment is that the car didn't make this noise when I bought it. Would an out-of-whack balance shaft make this noise as soon as the PO screwed it up, or would it present itself gradually as this seems to have done?

Thanks again for all of the inputs! I might be frustrated if driving this thing didn't produce the ear to ear grin each and every outing!

944Ross

Did you confirm the vibe is there with the clutch in? (i.e., TT not spinning)?

EJZero1

Forgot to address that earlier question...

Yes, sound is consistent in the 3500+/- range in all scenarios...in motion, hard acceleration, easy acceleration, standing still, clutch out, clutch in, in gear, in neutral.

As indicated earlier, next approach is to pull the heatshield off the vehicle altogether. This will either confirm the issue or at least narrow the search.

Thanks again for all of the inputs. You guys are an awesome source of info and feedback for this newbie. Will update with latest findings as soon as I have time to crawl under again.

75ohm

Before you go pulling the heat shield off, which is a bear to get back on (for me anyway), get a prybar or something and hold it in place while the wife unit nails up the rev to 3500...

FRporscheman

+1

Also try the exact same thing with the heat shield around the exhaust headers (just aft of the oil filter).

944Ross

I've had a cat conv (monlith type) batter itself til nothing was left of the honeycomb inside but a piece the size of a baseball. It rattled around in there for a couple months, then got stuck in the outlet of the cat and stalled the engine.