Posting this as a request for a shop owner who's been working on this problem for months w/o solution...

Car is 996 turbo. The engine has a loud humming noise (like that of a bad bearing) from the crank case, from the center crank shaft. The sound was isolated using a stethoscope.

All the copper sleeves on the shaft have been replaced, along with other replaceable/wearable parts.

He finally swapped the whole crank case from another TT engine, the noise is gone. But he can't figure out why...

The shop-owner said, he's seen a few TT with the similar noise as they age, but most car owners simply ignore it.

Any inputs? thanks.

 

Did the mechanic inspect the bearings?

 

yes.

 

^ the pinion/thrust bearing(s) will one day wear out. they dry out after enough heat cycles.

 

I just replaced my thrust bearing. It was only making noise under load. Foot off the gas - noise stops. It's a dead giveaway that the bearing is at fault. If your hum is full time and not limited to acceleration, it's not the thrust bearing.

 

The ball bearings have been replaced with new. So it's not from the bearing itself.


The noise is apparent from idle to about 3000RPM.


The noise is gone when the major components in the crank case (incl. the shaft) are swapped with another spare.


Any inputs? Thanks.

 

It has to be either a bad new part or worn crank.

 

A worn crank shaft means it's basically off-balance then?

 

Usually a worn crank will be noisy and or lower than normal oil pressure. If possible I would work a deal out with shop to exchange for lower crank assembly that is in car with new bearings etc.