When my 88 740 GL is in drive I get get a rumbling sound from underneath. Cant really pinpoint it. Either from under the floor pans or up front more near the engine. Even in park if I race the engine (tap the gas pedal)that rumbling sounds comes on but goes away once the car idles or shifts into the next gear (if in drive) I checked the exhaust and thats not it, I change the U Joints and Center support bearing, I change the transmission mount (which caused the problem to disappear for a day or so) but now its back. Im not sure whether while changing the transmission mount I moved the rumbling problem away from the cause and now it reseated again.