I have a 2000 Z3 and there is a thump that comes from the speaker

I have a 2000 Z3 and there is a thump that comes from the center speaker. The noise is there when the button for the power windows is used and when the car is started and turned off. 1 person said it's the main amp in the trunk, another the speeker, is there a way to test what it may be. The noise also sounds like a blown speeker however the speeker sounds ok at other times. The thump is not just playing music, it is there when i listen to talk radio also.