WallyP

Look on the brake booster and find the small black/blue check valve. Unplug the valve from the large brake booster check valve and plug the nupple on the booster valve with a short hose and plug.

If the noise goes away, you have a leak in one of the HVAC actuators. The black/blue check valve will sometimes hum or whine if there is a leak somewhere in the system.

928 at last

OK, on the assumption that a "turbine like whine" is being generated by something spinning at a greater RPM than the engine itself, and that no-one has secreted a turbo or S/C under the hood, also as the crew here has rightly identified, although an accessory is not the culprit, (I would have voted for the alternator and air pump theories first) my thought would be that you should check cam chain theory, but it's most likely a bearing heading south which once the accessories are eliminated, doesn't leave much to go after other than the unpleasant options of the water pump and oil pump. Trick is to at least isolate the source. Top of the engine or front of the engine? That would at least cut down on the options although it seems you've covered the obvious possibilities already. If the whine pitch changes in synch with the engine RPM, that's what I'd do, however if there's lag or the sound of something "spinning up or down" not in synch with the engine itself, I'd be at a loss for a guesstimate as everything except the fans on a normally aspirated unit is directly driven by the crank.
Good luck with this one.
Rob.

928 at last

If it's not rotational, +1 on what Wally just posted. My check valve was making strange noises prior to replacement.

Erling G-P

Thanks again guys.

Bruce: Air pump is removed on my car, and all the remaining belts were removed today, not just the alternator.

MrMerlin: Mechanic himself suggested maybe the changed oil in connection with the cam chains could be the source, but at 10W40, the new oil should be thicker than the 5W30 it replaced. Did notice the oil pressure drop a little when the 5W30 was put in last year, but I don't know the weight of the Liqui Moly it replaced.

MrMerlin/Wally: Could a possible vacuum leak result in a noise that rises and falls in pitch along with engine rpm, or would you expect it to be constant ?

Edit: Thanks Rob. Water pump is brand new, and the noise was there even with the (not-so) old one, so wouldn't think it's that. If the sound is present, and I slowly increase the revs, it seems to follow closely, but can't say for sure if it's exactly in sync.

Regards,
Erling

Mrmerlin

most likely a vacuum leak or bad vacuum valve or vacuum pot in the console

Erling G-P

Mechanics have been busy today. They're very certain the noise is coming from the area around the waterpump, and with it being replaced and the noise present with both old and new pump, suspicions now centers around the oil pump. They've asked me to come by for a listen myself, which I plan on doing tomorrow morning.

Have been looking for 'oil pump' at a Swedish Porsche parts vendor and at 928 Specialists, but can't find a single piece item with this designation. Swedish vendor seemingly has nothing related to the oil pump; 928 Specialists lists a number of components (Front Seal, Sprocket, O-Ring, A/T Converter to Oil Pump Seal and some tools).

Does anyone know if that is what's needed to restore the oil pump, or is there more involved ?

Regards,
Erling

Ben Allison

928 107 008 02 is the oil pump itself.

Bill Ball

These noises transmit throughout the motor and can even be amplified by resonant chambers away from the source. We had an air pump that generated a noise that transmitted down the bracket and was very loud right behind the water pump. I know you've already done the trick of removing the belts to rule this out, but my point is locating the source can be confusing. Still, the loudest point should be at the source. So, try to get a stethoscope right next to the oil pump and then listen at points away from the oil pump. You could try a mechanic's stethoscope, which uses a solid rod and is great for picking up mechanical knocks and clicks, but bearing-type whine might be better picked up by something more like standard sound gathering stethoscope. I modified a mechanic's stethoscope so it could use hoses or the solid rod. I don't think the oil pump is a likely source, and it's a chore to replace, so I'd want to be darn sure. More likely is a timing belt tensioner roller bearing, so make sure those are all ruled out by very careful listening.

Erling G-P

Ben & Bill, thanks both.

Bill, I believe they've been using a mechanic's stethoscope like you describe on it. All timing belt rollers have just been replaced without any impact on the noise, so doubt it's one of those unfortunately.

Regards,
Erling

NoVector

I'm trying to remember any mechanical failure I've experienced (like a bearing) and had it result in changing "pitch" as you mentioned in your first post. Seems like they just got faster and louder--but the pitch of the noise didn't change. Makes me think vacuum or maybe pressure from blow-by from a loose oil cap or a disconnected vent hose or something--that would also coincide with the first routine oil change maintenance visit.

Bill Ball

I know the rollers were replaced. I'd still listen to them very closely.

figgen

does it make a difference when the clutch is engaged or disengaged? just a thought - pilot bearing or throw out bearing ?

SharkSkin

I agree with Bill, check the rollers again. Any manhandling of the rollers getting them on could damage the bearings.

Bill, can the 32v motors run with the center timing cover removed, or do the cam sprocket covers not fit properly without it?

Mrmerlin

32 V engine has index pins on the rear cam covers so the center cover can be removed after removing the crank damper. Then refit the cam covers with distributor caps

SharkSkin

In that case(it should be obvious where I'm going with this) they should run it without the center cover, unless they diagnose the problem some other way first. Using a rod-type stethoscope they can check the oil pump directly, along with the other rollers.

Have them check the various plugs and fittings near the oil filter and cooler lines. One of the pressure/bypass/thermostat valves may be squealing.

Also -- this would be unrelated to the work that was done IMHO -- if the o-ring on the oil pickup decided to let go, you could be getting air in the oil system which could make a noise similar to what you describe. I haven't experienced this personally, just stretching my imagination a little.