Tingle/buzz in steering wheel at idle

[S-Class Guru]

I'm getting a "tingle" in the steering wheel while sitting idling in Drive or Reverse. Been doing it over a year now. Feels like a vibrator on slow speed. Can't feel it in the shifter, can't really detect it much through the armrests or dash or floor. Goes totally away if I rev it just a hair above 600, or if I shift to neutral. Did all the usual suspects - tune-up, injectors, motor mounts, little motor mount shocks, exhaust hangers, motor touching frame/etc. Did the tensioner shock. Even took the belt off and ran it a few seconds - still did it. Motor is running really smooth, compression is pretty equal on all cylinders.

It's so consistent, seems to vibrate at the frequency of the motor RPM (Buzzzzzz). So I've kinda ruled out a miss or a bad injector at that idle speed.

I'm thinking maybe something in the torque converter or tranny front pump - but just cant feel a thing thru the shifter. I did loosen and retighten the flex converter bolts.
Next suspect is harmonic balancer - seems to run straight though. And as I said, rev it from 600 to 700, or shift to neutral and it virtually goes away.

Anyone ever experience this on the 103 engine ('91 300SE)???

DG