Finally figured out what was causing my front doors to rattle when playing music with heavy bass.
Turns out I didn't have to remove the door panels to fix the problem and my solution seems to be holding up.
Tools needed NONE!
When I was investigating the annoying rattle I noticed that if I pressed against the door panel the rattle would go away. I noticed that the door panel has an outer shell that fastens on top of the inner shell. If you run your fingers inside the door pocket you will notice where the inner panel touches the outer at various locations.
My fix was to insert small wedge shims (the ones used to install tiles sold at Home Depot) between the surfaces (directly below the speaker grill where the cup/water bottle holder is) in order to slightly separate the door panels. Since the surfaces aren't vibrating against each other the annoying rattle is gone. Sweet!
Hope this helps others experiencing the same problem. By the way I have the six speaker system complemented with a small sealed enclosure with two 8" Pioneer shallow subs with a tiny Rockford Fosgate digital amp. Bass is surprisingly decent.