Check your transmission mounts, they rot, and when the car idles and it heats up under there, the rubber gets soft. The rumble may be the driveshaft hitting the cat heat shield. Also check the driveshaft center bearing while you are down there.

These are both pretty easy to fix if you get the car up in the air..

our 88 735i drives fine almost all the time, but should i drive the car then let it idle for a short time, or turn it off for a short time like to run in and pick up a friend or something, when i get back into the car, as i accelerate away, there is a loud rumble. at first i thought something was banging on the wheel or something, but it doesnt makle the sound while the car is coasting, only accelerating. any ideas?