I have this same rattle...seems to vary with temperature. Much more pronounced now that its cool. Took the inside of the trunk liner out partially to take a look. There is a small hole in the metal deck in the same general area of the rattle. I'm thinking about drilling the hole out....making it a bit wider and then stuffing some high quality foam into the opening. I don't like the idea of the dealer disassembling the back seat to get to the rear deck...probably end up with three more rattles.