Your rear window likely needs to be resealed. Try 'knocking' on it, if it rattles at all, you'll need to redo it. The rubber seal runs along the outside bottom of the window, only, and doesn't serve to hold it in place. The window is affixed with butyl tape, you can pick up a roll cheap at your FLAPS. There's an excellent step-by-step installation guide in the 'classic threads' section of the site written by Eric Shea. I've done this myself twice, not exactly fun, but it's nice when you're done ~ no more rattle!
Good luck. Oh, and this should be posted in the garage forum.