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.