My front passenger door speaker rattled like CRAZY at low bass (stock speakers), so I decided to investigate. After getting the info from jcaputo on how to remove the interior door trim (thanks! ), I think I found the source of the problem.
The speaker is in the "front" (towards hood) of the door, and there is some kind of plastic box in the "back" (towards trunk) of the interior door trim. After playing some music and knocking around, it seems that the plastic box in the door trim is rattling against a plastic lattice in the door itself. It's supposed to butt up against it I guess but it's juuuuust separated.
See pictures to clarify -- I didn't have any dynamat or similar on hand, and just wanted to try and find the problem first so I used duct tape and packing peanuts! I first braced the plastic box against the door trim so it itself couldnt rattle, then put peanuts on the outside where it mates with the door frame plastic lattice.
Now all I hear is the rattles coming from the OTHER 3 doors but this seems to have improved the front passenger door