The car is an '89 S2, 140K on chassis, 1500mi on rebuilt motor. Aftermarket intake with a big K&N cone filter behind the header panel, Weltmeister clone chip, mostly stock everywhere else.

The noise is hard to describe - it's a bit of a hum/howl that happens consistently between 2K and 3K rpm after the car warms up. Once warm, the noise is very consistent in that rpm range, and the tone of the noise never really changes - it stays the same pitch.

The noise started after I had some work done to the car at a local shop - t-belt/tensioner, oil pan gasket, and radiator. I heard the noise on the first drive home, and it's been there ever since. It does not seem to affect performance, as the car performed flawlessly at a DE event in June.

What I've done:
- Been through the engine bay looking for loose hoses, specifically vacuum hoses
- Removed the aftermarket intake and reinstalled, verified that all hoses in the intake tract were snug and clamps tight.

I'm at a loss on what to look for next. All suggestions welcome.

 

It could be your t belt humming... Mine is pretty loud also but all around at idle and moving. But I would think your clutch or your transmission would be making some sound like this if I am thinking of the right noise.

 

The transmission makes its own brand of noises - it has a bad bearing that makes a good bit of racket. The clutch is new, installed at the time the motor was rebuilt. I've asked some local folks and everyone is leaning towards the t-belt/balance shaft belt so far.

 

That could very well be it you should definitely have that checked out....

 

A little update. I had a local shop take a look at the car (for free ) on Friday, and he didn't find the problem, but did find out a few things:

- Timing belt is OK, timed correctly, no contact with anything, and has correct tension.
- Balance shaft belt is OK also
- The exhaust doesn't have the normal amount of "play" that should be allowed by the rubber hangers.
- The car has 2 different motor mounts in it - one an OEM replacement and one aftermarket piece. The OEM mount is a little taller that the aftermarket, so the engine is canted just slightly to one side as a result.

The issue is definitely thermal. It consistently starts after about 10 min or so of driving - enough time for the car to get up to operating temps. My mechanic's theory is that the exhaust system (likely the cat. con. itself) doesn't have the clearance it should, and when hot, expands enough to touch the unibody. The 2K-3K rpm range must create some kind of harmonic that makes the noise. The clearance issue is likely exaggerated by the different motor mounts and the "tight" exhaust.

I have a test pipe that is a smaller exterior diameter than the stock midpipe, and I'll put that on before the next track day, and attempt to move the center exhaust hanger around a bit to see if I can get some more "play" in the system for more compliance.

 

Very interesting! I have the exact same phenomenon on my -91 S2. It's been there since I bought it a month ago and I'm very curious in your findings. I thought it was noice from timing belt/ balance shaft belt or tensioner. I will follow this thread closely...

Rgds,

J

 

Unfortunately, I'm not exactly sure when I'm going to be able to put the car on jackstands and mess with it for a few hours, but the more I think about it, the more this theory makes sense.

At the end of the midpipe, there's an exhaust hanger that's actually a long ubolt that goes up over the torque tube. The first time I swapped the stock midipipe with the test pipe, I messed around with the ubolt hanger quite a bit. I'd likely moved too far towards the front of the car, and the nuts are likely too tight. The end result is that it may be holding the midpipe too snug to the unibody. Since I also get some "buzz" that is clear contact between the exhaust and the unibody at certain rpm/certain situations, I know the exhaust needs some tweaking and alignment.

Man...it's going to be really disappointing if this doesn't work.

 

Any news yet? When I was checking this just briefly (with hot engine) I would just lean over the engine slightly and and rev it some to locate the humming sound. Definitly sounded like it came from the front of the engine or under the belt cover somewhere in my case.

/J

 

News, but not of the good variety.

I finally got around to swapping out the midpipe to my test pipe this weekend, and I thought I had found my problem. The u-bolt that hangs down from the torque tube and acts as the middle hanger in the system - it was way too tight and also off center, pushing the exhaust over to the passenger side of the car. When I looked at the heat shield, there was definitely contact between the cat section of the midpipe and the heat shield.

I moved the u-bolt to the correct alignment, installed the midpipe, and took it for a test drive. When the car got up to operating temp, it still made the same noise between 2-3K rpm, albeit slightly less loud this time.

At this point, I'm in "just live with it" mode. It's an 18 yr old car, and this seems to be a benign idiosyncracy. The car runs VERY strong and all the vitals look great (oil pressure, water temp, voltage). I'll be at Summit Pt next weekend with NASA to drive the car how it was meant to be driven.

 

not sure what you mean by "hum" exactly, but the oil pressure relief valve can hum when it starts to get old. but it's mostly heard at idle.

 

The noise is hard to describe, but this is the best analogy I can come up with:

Imagine one of the old glass 16 oz soda bottles you used to get as a kid. Take that empty bottle and blow over the top of it, at just the right angle until you get that little resonance. That resonance is very similar to the hum I get.

The pitch of the hum doesn't really change, only the volume. It starts quietly at just over 2K rpm, seems to be the most loud at 2500rpm, and tapers off as you head to 3K rpm. There is no change in drivability (no stumbling, etc) or power when it makes the noise. It just makes the noise.

 

Could it be the throwout bearing? Does it stop if you tap the clutch pedal?

 

I've never tried to replicate the noise with the clutch pedal pressed in. I'll try that on the way to Summit tomorrow morning. I think the throw out bearing is new, but I'd have to check the receipts from the previous owner to make sure.

 

Whilst driving to Summit on Saturday morning, I pushed the clutch in and held the revs at 2500. The car still made the same sound.

After putting ~150 street miles on the car this weekend going to and from the track, I still stand by my impression that the noise is not as loud as it was when the stock midpipe was on there.

The car ran flawlessly all weekend, even with the near 100 deg air temps and ~70% humidity.

 

Bump...

I'm interested if this noise problem was ever solved? I have a similar noise when applying the accelerator gently from coasting to accelerating. It is almost like a scraping sound too. I had my '00 996 Carrera 2 at a Speed shop yesterday and my mechanic thought the sound was coming from the axel bearings by the differential.

I do not hear the noise if in neutral while coasting at any speed or when revving the engine without the transmission in gear.

The noise is more pronounced when in a high gear (i.e. 5th) and driving at 50mph.

The noise occurs only after/while applying the accelerator from a coasting in gear.

Any help is greatly appreciated.

Best regards,

Markus