Speakers on BMW 328i, work fine when standing, but once start driving give out a knocking sound.?

I have a 2000 BMW 328i, and the original factory subwoofer (2 piece) in the trunk. When the car is standing it plays fine, but once I start driving there is like a loud knocking sound. All the other speakers work fine. Does anybody know what the problem could be?

Speakers on BMW 328i, work fine when standing, but once start driving give out a knocking sound.?computer

If it changes with engine RPM's, a noise filter(inductive type) or ground loop isolator may fix the problem.

Speakers on BMW 328i, work fine when standing, but once start driving give out a knocking sound.?free spyware

they may just be loose
I'm not sure what you mean by standing? With the engine on but the car not moving does it make this sound? My first guess is to check your spark plug wires. This sounds like a suppression problem. If your spark plug wires are old or one is loose then you will sometimes pick up a popping in your audio. Make sure that your subs are to the frame (probably not the reason).