worf928

Expansion from heat.

I think so too.

It seems logical to assume that something is slightly loose when the motor is cold. When you put the car in gear, the change in vibration frequency from idle sets the loose part to rattling. Once the motor is warmed a bit, the loose part expands and its resonant frequency changes such that it doesn't rattle audibly.

Bill Ball

I had the listening hose between my ear and the bottom of the bellhousing cover when Nicole started the motor, as that was one of my concerns and Nicole thought the noise was coming from under the car. It was quiet. I could hear the noise outside the hose coming from way forward. I think the fleplates (front and rear) are ruled out, but we will do one of our regular front flexplate position checks when we get the car over on the lift at my house and make sure all the bolts are secure at that time.

Bill Ball

Yep, could be the air pump. I wasn't quick enough to get the listening hose on it. Nicole was doing some of the hose work. Maybe she listened in that area.

Mrmerlin

if you dont find anything, then i would check the securing nut on the oil pump shaft and the crank shaft nut. timing cover off for the oil pump check

6.0-928S

This sounds crazy but it happened to me. The sound you describe is exactly the sound I heard from my 928. It also went away when the car warmed up. I put belt dressing on the belts & the noise was gone! I know that it sounds metallic but in my case it was cold belt flex. When they warmed up they were more pliable & the noise went away. While it is running & the noise is there, spray some silicone on the belts & see if it goes away.

Hammer

IcemanG17

Hmm
The fact that it only does it when the car is cold and when placed in drive, lowering the rpms....but it goes away quickly once the car is warm makes me think it is NOT an accessory like the airpump or alternator.....

But I can't think of something that would "warm" up in that area that would make that kind of noise....like the common ticking lifters most 928's have when cold....

If the rpms were to go up, does the noise change pitch, get louder or go away?

Tails

Bill,
Your are correct, there is no rpm adjustment on the LH in the late model cars that I have found or read about (John from JDS can confirm this?).

Another possibility is that one of the pullies on one of the ancilliaries is loose or the shaft is worn or if they are "key" on the shaft and the key is slack, the pully could be suttling or rattling until the engine or the pulley settles down to constant rpm's as the engine warms up.

It could be a case of removing all ancilliary belts and check out the pulleys, driver and driven.

Another item that rotates and could rattle could be the electric fan blades or their mounting in the shroud. Is the A/C switched on when the rattle occurs?

If it is none of the above then it could something that is just touching another part that rattles at start up or is slightly loose and the loosness it taken up as heat from the engine perminates through the engine bay.

I'm now getting to the stage of telling you how to "suck eggs", so the best of luck with your search, it will be interesting to know what it is.

If it is none of the above then it could be into the next belt and rotating area the TB!!!!!

Tails 1990 928 S4 Auto

Nicole

Great ideas! A few things I might be able to check before I get to Bill's garage in two weeks.

It's not dependent on whether AC is on or off.

We've had some issue wit whining from the Alternator belt before, which a BOSCH rebuilt Alternator did not fix. However, I don't think this is related.

Randy V

I've had the EXACT same thing for almost a year now.

Post a follow-up if you figure it out!

Bill Ball

Yes. Removing the belts and adding them back one at a time until the noise is present might be what we have to resort to if we can't find something loose or track the noise to its source with a hose.

Tom. M

Has no one mentioned Rod knock...I think that is what rod knock typically sounds like..???

Bill Ball

Hammer:

I missed this earlier. Funny you should mention it. Nicole's PS and air pump belts are rather loose. The PS belt runs very close to the top mount bolt for the alternator. I pointed it out and told Nicole we would snug up her belts when she comes over. That may be all it needs. Thanks for reminding me.

Bill Ball

Rod knock is a deep sound and is more regular. She does have some start-up rattle in the drivetrain the disappears in a few seconds and could a lot of things. Maybe lifters, although that's more of a loud ticks. To brief to track down easily. I hear it in a lot of 928s.

IcemanG17

Bill
Its an easy test for belt squeek....just spray a touch of silicone or WD40 on it..... My saturn did that really bad.....dealer sprayed some WD40 on it...went away, just to come back 10 minutes later...

FlyingDog

Exhaust manifold heat shield? Does that sound go away when the idle drops when warm or it just fades away as the car heats up. If it fades, I'd say expansion cures it. If the idle change does it, I'd say it's harmonics.