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?