I'm stumped. My wife's '98 E300 TurboDiesel has a deafening rumble/shaking when sitting still in drive at a stop light. It only does this in a forward gear. What's weird is that it runs so smooth and has no other symptoms which would help me target the problem. BUT, when I move the gear selector into Reverse, the sound completely goes away. Idle speed is dead on at 600 rpm. Any insight would be greatly appreciated.

 

sounds like your engine mounting has collapsed. you will notice it sits lower on the bad one compared to the good one.

 

thanks g. i went ahead and got some mounts ordered and on the way. after doing some additional research, i performed the "turn the wheel to the left with the hood up so you can see the motor and step on the accel pedal" test to check for vertical engine movement, and sure enough, the engine rose up about an inch on the left side. looks like the driver's side mount is shot. lucky me, i've got a local guy who spent 12 yrs as a mechanic at an MB dealership who is rea$onable on labor.

 

changed out the motor mounts and the car is very quiet now. WOW! what a difference. my wife is waaaayyyyy happier now.