In reply to: Do you see/feel/hear any other indications? (I haven't yet.) How long does it last? (Mine is seldom more than 30 sec.)

YES! I was sitting in my driveway last night, after arriving home. I placed two fingers on the gas pedal to confirm a rather unpleasant strong vibration. It was like a little motor is running or something but immediately stops when placing the transmission into neutral

The 1991 560SEL has a very strong and steady (gas pedal) vibration, after driving for a while it will intermittently, yet systematically come on, "run" for 20-30 seconds. When its real quiet outside, I can hear a faint buzzing sound coming from (a hydraulic pump or fuel pump?) under the (front/ or back?) of the car, directly associated to this vibration, (I've not located the source yet). The pedal vibration and buzzing sound start and stop together (…after completing some task?). This strong “buzzing sound and pedal vibration” occur only after driving for awhile and are noticed best when stopped, in drive, w/ brake on, and applying pressure to the pedal, it’s easy to tell. It’s like a foot vibrator; the vibration goes directly into the ball of the right foot. Pushing the gas pedal further down, (just a bit) and consistently the vibration and buzzing sound immediately stop. This is a repeatable pattern. It doesn’t seem to be connected in any way to the (my) cruise control. My cruise control works just fine, all the time, with out surging.
This again is rather new. I’ve had this beautiful sedan for almost 2.5 years, the vibration just started three weeks ago.
Recent changes:
· Replaced front pump and lip seals on transmission, to fix reverse clutch. Reverse had gone out completely.
· New Motor mounts;
· Accumulator Air cells (rear levelers) replaced;
· All Engine belts replaced;
· Leaking right V/C Gasket replaced

Please excuse this long post, your ideas/ suggestions are very appreciated : )
Thanks.

__________________
Todd