Any help would be appreciated with this ongoing problem I'm having with my '89 420SEL:

Intermittantly, while the car is in motion (seems to only occur at speeds above 35MPH, though most frequently at highway speeds), something under REAR LEFT of the car whines quite audibly. It sounds exactly like a loose belt under the hood, but the sound definitely emanates from the rear of the vehicle. (My 7-year old has taken to calling it "the mouse under Daddy's car...)

The whining lasts anywhere from 1 second (a "chirp') to 15 seconds every time it occurs. That is, unless I let off the gas, in which case the sound dies down quickly. I also noted during the summer that turning the air conditioning on and off would sometimes start or stop the "mouse".

Final observation: the cold weather here in New York has definitely reduced the frequency of the squeals, but they still do occur.

All fluids are clean and filled; and the car is generally well maintained.

I suspect the tranny (or possibly the differential) given the location from where the sound emanates, but I really know very little about the innards of these systems. Shifts are a little rough, but no more so than before this problem started.

Has anyone experienced similar symptoms? I would love to know what I am dealing with here...

__________________
Eric

1991 560SEL-Euro (214,000 miles)
1989 420SEL (Retired from daily use at 325,000 miles; Use as donor vehicle)