I wonder if you were to unhook 3 speakers, and then take the door panels off all 4 doors. Play the music with the bass and physically see & touch the innards of the door as it's playing.
The vibration may be just the entire door structure vibrating to the music, and not a loose piece or rattle.
But, if you can find it and by touching it or pressing against it, it goes away you might be able to add some foam padding or something similar to press against it when the door panel is replaced.
Just a thought
N.
