I had whining coming out the transmission before. It turned out to be front pump bearing surrounding torque converter. Somehow after rebuild it would not have enough play for torque converter and it would whine. I bought used tranny and after 1 week it started leaking and making very loud noise. Took it to a shop, took it off the car and it turned out that same front pump bearing spun inside the pump making too much play. Shop owner personally replaced the bearing with new one and no tranny is perfect. All that info was given to me by shop owner who rebuilds trannies. He doesnt rebuild MB trannies but he said that all transmissions have those front pump bearings and they are one of the main whining noise causes.
You could have something else wrong with your tranny but check the bearing as well. It cost me $2 but took day and a half to take tranny out and partially disassemble it. I did it myself and my car is 92 500sel. Yours should be easier since its inline 6.

Good luck.

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1999 ML430 Silver 135k ml
Smoked LED taillights, Alpine in-dash DVD player, 12" sub, Pioneer components, 2 amps, 4200k HIDs, ML55 headlights, 20" rims, mirror LED blinkers

SOLD. 1996 E320 145k ml. Burgundy

SOLD but miss it everyday 1992 500sel Black
Chime buzzer mod,Alpine CD/MP3 player with stock amp,Euro headlights,S600 grill,20" rims, BEST CAR EVER!

SOLD. 1990 300E White
187k ml,smoked tails,Euro headlights,17rims,Intake,12" sub in first aid kit,alarm/autostart,buzzer mod