Just a guess but i'd say there's something loose in your door.
Seriously, take the door panel off and see what's causing the rattle. It's probably something small like a lock rod or something that fell through the window rubber. You could put on a little Dynamat and maybe cover the rods for the doors with rubber hose. I haven't taken off the inner door for a G yet but it can't be that difficult. Get yourself the Factory Service Manual and i'm sure it details the procedure.
-Grant