Luis de Prat

I recently replaced all belts and the 5 rollers on this car. Driving it today I noticed it runs great, but that there's a momentary vibration/rattle noise when accelerating that stops as acceleration increases. The sound is not loud or anything, but clearly audible.

When tensioning the belts I got both of them spot on the ideal setting per the Arnnworx P920X and I just did this a few dozen miles ago, so it can't be time for a retension.

The vibration only ocurrs when the car is in motion and not from revving the engine when sitting at idle.

Any ideas?

harrisonrick

Hows the clutch? Rubber-centred?

Luis de Prat

No, the clutch was replaced with a factory spring centered unit less than 30K miles ago. The car shifts and drives normally, with minimal shift lever effort. Thanks!

Eric_Oz_S2

Luis, you might check that the belt covers are on securely and that the rear cover is not vibrating against the engine. I have a small piece of rubber separating the rear belt, presumably installed by a PO some time ago to stop vibration here. Just a thought - noises are hard to diagnose without being there to hear them.

Luis de Prat

Thanks for the thought. I will check this. I'm leaning towards the differential being low on oil.

Luis de Prat

OK, I stopped by the shop that sometimes works onthe car this afternoon and they really doubt it's the differential since the noise is coming from the front.

The clutch was replaced at 138K miles and the car now has 158K so it would really suck if it were failing now. Especially when I went through the extra expense of putting in an original 944 S2 clutch from Porsche.

Any other ideas?

Makis

Does the car have the vibration at a fixed rmp. My S2 has a small resonance that is causing a faint rattle at around 3K. And this happens at any acceleration and gear. So is a engine or perhaps torque tube induced resonance. I replaced the belts and the noise did not disapear and have double checked the balance belt so is unlikely to be the belts.

Luis de Prat

How interesting. Does yours' resonance go away as acceleration increases? Mines' does.

Makis

I guess if I accelerate hard in 1st gear the revs will pass past 3K so quickly that I won't be able to trace the rattle. When the engine revs up and reaches 3K rpm the resonance can be heard. Once the revs go to 3.1K or 2.9K the resonance disapears.

JohnKoaWood

Starter heat shield, that is where my money is...

Or possibly an exhaust heat shield...?

not loose, but vibrating... try hitting with a rubber faced hammer and see if you can hear it... the rattle that is...

Luis de Prat

Thats very interesting too, since I dropped the starter to install a flywheel lock when doing the belts, but I did not see a heat shield. Does the 944 S2 have one?

Mark944na86

Mine does.

CVambience

My 1991 S2 does exactly the same thing, that vibration at 3k rpm, have not yet figure out the cause, would love to know why.

Luis de Prat

Well it wasn't doing it until a few days ago, at 159K miles.

jstand22

something like this is often balance belt related....
the tension may be incorrect on the balance shaft belt, or one of the balance shafts may not be aligned correctly.