Call me crazy, but I dropped the truck off for what my dealer calls the wihter package. Includes undercoating, leather protectant, and paint protectant. Gives lifetime warranty on underside, and 5 years on paint and interior. Also had clutch packs in rear replaced. Ever since this was done, WTF no more rear window rattle. Quiet as a mouse, and mine was loud before. Have driven quite extensively since I left the dealer, about 1500 miles, still nothing. I leave this one upto you all to figure out because it has stumped me.

Adam