When I took my door-trims off, I banged on the door and listened for rattles. Half of it was from the door trim itself! Up and under the inside of the trim, where it sits on the window-sill, there are metal sort of hangers/reinforcements.
I had some strips of velcro spare, so I used the furry side, and cut them to length. Now it sits tighter, and hasn't made single sound!
Also, tighten the door-skin... speakers don't need much volume to make that stuff shake, and it sounds like there are shopping bags in the door if they're loose.