My '91 300E has developed an unusual (to me, anyway) vibration. It's nice and smooth all the time except: when traveling at 45+ MPH and the engine is pulling pretty hard. IOW, you're driving on the highway. All is fine. You start up a hill. The vibration starts. Take your foot off the gas and it stops.
It's not dependent on engine speed. When the vibration starts, if you pull the shifter back to 3, the vibration stays the same.
It's not really like out-of-balance wheels. It's more like a rumble throughout the car.
The motor mounts were just replaced and the transmission serviced. I noticed the vibration the first time on the highway after that. A coincidence, perhaps.
Ideas and opinions?
Thanks for your help.