Erling G-P

Hi everyone,

Have a bit of a headache with my car and hope someone can provide some suggestions or ideas.

Will try to make a long story short, so here goes:

Had regular minor service done by my usual mechanic; basically oil/filter change and the brake fluid was changed too.
Oil was changed from a 5W30 to 10W40.

When I picked the car up after service, I noticed a new noise in the form of a turbine-like whine that follows engine rpm. Sound is most audible from the driver's seat, with windows closed. Can be heard too with the hood open, but darn difficult to locate exactly where it's coming from. The sound comes and goes; sometimes you can hear it, only to have it disappear like a switch was flipped. It's most audible when you change revs rather slowly, so you can hear the pitch change.

At the same time, I started getting 'cam belt service' warnings from the dash computer. Naturally assuming a connection, I arranged to have waterpump, cam belt and rollers changed. Turned out one of the roller bearings was very bad (despite apparently being replaced by the PO less than 3 years / 26.000 km ago)

No more warnings so far, but noise still there after this. Had a few other 928/944 guys listen to it at a meeting and the general opinion was that it wasn't internally from the engine, but rather the alternator, servo pump or perhaps just a belt making the noise.

Had my mechanic remove the belts today, and it didn't remove the noise
He's used a listening device on it and it still sounds like a bad waterpump, seemingly coming from the area around it, but since sounds wander, it's very difficult to say with any certainty.

Could it perhaps be the oil pump ? Afaik, it's located near the waterpump. Mechanic is reluctant to make recommendations. Could start replacing oil pump components, but as he says, that's basically doing the whole cambelt operation once more.

At one point when I myself was trying to locate the source of the sound, I thought it sounded loudest around the top of the engine; near the inlet manifold, and wondered if it might be a whistle instead, perhaps from false air being sucked in somewhere, but mechanic was quite sure it's rotational in nature.

Any suggestions and ideas are most welcome, as I'm feeling really uneasy about this.

Thanks in advance
Erling

SharkSkin

I have not heard of oil pumps going bad on these cars, but I'm sure it can happen. Were any of the bearings in the timing belt path re-used?

Listen near the center of each valve cover -- could it be the cam chain(s) have worn through the plastic blocks? What is the total mileage on the engine?

Check for shredded rubber on top of the air filter and/or inside the timing cover(look inside the fitting at the top of each am cover where the intake tube attaches). Could be the belt is not tracking properly.

Does the sosund change between startup(cold) and hot?

Erling G-P

Dave, thanks for the reply.

The cam sprockets weren't changed and neither was the oil pump sprocket, but all other rollers should have been replaced.

Mileage is 193.000 km (120.000 miles), but half the engine was replaced at 50.000 km (31.000 miles) because of excessive oil usage. I assume it must have been the block they replaced, but don't know for sure.

I haven't noticed a hot/cold difference as such, but since it comes and goes and is obscured by other engine noises too, it's difficult to say really.

Regards,
Erling

Mongo

My alternator made the same noise before crapping out on me.

Mrmerlin

the oil pump could be put together wrong anf this could cause it to seize, the outer gear can be positioned wrong.
That said I might suspect one of the tension rollers in your belt drive. Pull the dipstick and drip some of the oil onto black cardboard go in the sun and see if you find metallic particles. If so then it might be wise to open up the oil pump.
There is also a chance that the timing belt could be rubbing on the inside of the time cover, look for a polished edge on the belt

sidehatch

I had a whining noise that stopped after the flex plate was relived of pressure.It was the torque tube making the noise,when it returns = screwed.

dprantl

Also check your timing belt tension...

Dan
'91 928GT S/C 475hp/460lb.ft

Erling G-P

Thanks everyone for the suggestions.

Andy: Mechanic removed alternator belt today, and it didn't remove the sound, so it can't be that.

MrMerlin: If the oil pump is put together wrong, it will have been running like this for at least 27.000 km / 17.000 miles. Would it last that long, or would you expect the problem to show up sooner ?

Sidehatch: Mechanic is very certain the sound isn't coming from the back of the engine, and it's a manual, so that rules out the flexplate as the culprit AFAIK.

Dan: According to my mechanic, they rechecked the timing belt tension today, and losened it briefly, but it had no effect on the noise.

Spoke to mechanic earlier this evening, after seeing Dave's reply. He found the timing chain worn through plastic blocks suggestion very interesting - and a likely source of the type of noise in question, so that's the lead they will follow next.

I myself seem to vaguely remember some postings about those plastic blocks earlier here in the forum, and do believe the car in question had a mileage more or less similar to mine.

Thanks again all for the ideas; I will keep you guys posted about the findings.

Regards,
Erling

Fabio421

I had a very high pitched whine that changed speed and pitch w/ engine RPM upon arrival at Rennsport reunion a couple years ago. Everyone there took their shot at listening and diagnosing. Most thought it was the PS pump. It turned out to be my bellypan. I had bottomed out the car slightly after fueling up on the way to the event and the belly pan had bent in enough to contact one of the pully's. It was barely touching but it was enough to make a heck of a noise. I know it may be a long shot but it's worth eliminating as a possibility.

SharkSkin

Good call Fabio... I had a strange screeching noise that happened at about 130mph. I pulled off and changed my shorts, then checked things over -- nothing. Started driving again, and at 130 -Screeeeech!

Eventually I figured out that the wind pressure was pushing the belly pan(short pan on early cars) up into the AC pulley at speed.

Erling, they may be able to diagnose without pulling the cam covers, which can cost quite a few bucks if you're having a shop do it. If they do have to pull the covers, and they get them back together without leaks cropping up, you will have taken care of a number of latent oil leaks that are pretty common on 32v cars of that age. Might be a good time to powder-coat the covers if you're so inclined.

Erling G-P

Thanks Fabio for the suggestion.

I actually got this idea myself a while ago, after searching the forum for posts describing similar problems.

It was really the perfect explanation on paper, since the mechanic adjusted the bellypan upwards during the service, but alas, when I checked, there was plenty of room between the pan and the pulleys unfortunately.

Today's experiment with running the engine without auxillary belts (so the pullys aren't rotating), without any effect to the noise, would seem to be the final nail in the coffin for an otherwise very likely - and wonderfully simple - explanation.

Regards,
Erling

Erling G-P

Thanks Dave. I have just finished preparing some documentation for the mechanic which I will hand im tomorrow and he will then go hunting with his listening device. Do hope he can pin down the problem without having to pull everything apart.

Have neither the skills nor facilities to do this kind of work myself, so have to rely on the shop doing it.

Regards,
Erling

Mrmerlin

the oil pump would have seized a long time ago if the ring gear was installed backwards

NoVector

I had a whine that sounded like a turbo (which was actually kinda cool). Turned out to be the air pump. I found that it was the cause accidently during my air pump delete when I converted to an electric fan. All of a sudden, no more whine...

You said that your mechanic removed the alternator belt with no effect; but, I'm thinking all of the accessory belts could be removed and then run the engine. It would: a) make it a lot quiter to find where the noise is coming from, or b) you'd discover the accessory that's causing it--maybe by putting them back on one at a time.

Good luck! / Bruce

Mrmerlin

since this happened after service i would question the oil grade, possibly too thin maybe low oil pressure to the heads so the tensioners may be making the noise.
Possibly a cracked vacuum line where its connected to the intake, The eductor could be cracked or pulled apart enough to cause a whistle since the brakes were flushed maybe the vacuum line was disconnected.
Possibly the little blue vacuum valve on the booster.
Possibly one of the diaphrams has sprung a leak, pull down the side console to test the vacuum pots.
Check any of the vacuum lines for cracks since the car was just serviced