In my '90 745T with AW71 transmission I have been experiencing a pronounced medium-high pitched whirring or belt-like noise as soon as I let off the throttle.

I can hear the sound from outside when the windows are down, (especially if there is a Jersey barrier or big curb) and it also seems to come from the forward transmission tunnel area from inside.

It is especially noticeable when in D, or engine braking in 3 or 2. It only occurs off-throttle; when giving gas it is ABSOLUTELY normal.

Coasting in N, the sound is very faint but still there (I think), if no throttle is given.

I have read some posts that suggest a noise while coasting might be caused by bad mounts, especially the transmission mount. BTW does that refer to the mount right under the tailshaft Seal? I think mine is on the way out due to a slow-leaking tailshaft Seal. Anyone who is familiar with that syndrome is the noise a high, whirring sound?

Thanks for your help! Any ideas are welcomed.

I changed the transmission mount while I was doing the overdrive solenoid-

The noise is gone - !

Thanks for your help.

Hi Bob,

I have a new rear 740 mount FCP # 1328900 $10

Bill

Since you have a turbo I would suspect the compressor bypass valve (CBV). It's supposed to dump intake turbo pressure when the throttle plate is closed. Your can test it by maually moving the diaphram and plugging the vacuum line. If the valve stays open it's OK if it closes then the diaphram is broken and the valve should be replaced.

Could be your tranny bushing- that makes sense with the symptoms of not when coasting...
I think (not done it myself yet) that those are pretty simple to replace
It might also be the Center Support Bearing, just back from the transmission. don't replace it yet but spray it with some lube/pry back the rubber to get lube into the bearings.

Happy wrenching!
K