very cheap and easy way is to put some sponge pads in the middle of the latch on the rear seats. I don't know what you can do about the door panel, but I have also experienced tailgate rattle. The way to fix that is put black tape on the latches of the tail gate. I went to the dealer to ask why this is happening and they said its due to its stiff suspension. I also checked out and test drove a brand new x5 and experienced rattling noise from the seat (not much from the tailgate). I hope this helps.

09 BMW X5 xdrive48i (sport package)