Cruising at 70 mph down the highway in my 240D. As long as I keep pressure on the pedel it makes its noraml diesel humming noise...when I take my foot off the accelerator it makes a weird low, bassy humming noise. Is this just the German engineered gearing pulling the car back and slowing it down or something else? (flex discs) Any audible ways to know if flex discs are bad? I cant get it up in the air until tuesday.

Be sure ALL your fluid levels are correct and check the play in your cv joints...many power train items make a very different sound in the unloaded mode... actually this would be a braking mode...

It sounds like a humming nosie from the back of the car. I apply the accelerator and it is fine again. And normal diesel hum. Let off the accelerator and a humming noise seems to come from the back of the car. Dont think the flex discs have been replaced since the originals, but I am not going to drive it too far. I will take it to the shop soon and have my friend thin about what it is.

*BUMP*

Kyle,
check your rear-end fluid. But with 313K this is probably not a sound to worry about too much as long as all fluids are up to correct levels. Worn bushings and the like could also transmit sounds you may otherwise not be aware of too. I might be wrong about this since I only used to check my Mazdas like this. The center drive-shaft bearing, get under car, grab axle and try to move it. If it's like the Mazda the axle's not supposed to have much flop in it.

Exhuast backpressure...let off gas and no more back pressure on engine...therefore the "humming" noise from the back of the car.