Sometime my 2004 Caravan's rear drum brakes will stick while parked (even with no parking brake applied, generally in damp weather). When this happens, they let go with a "clank!" (or "clunk!") as I start to back up. When new, this was so bad they were "fixed" under warranty (apparently the factory screwed up something in the original rear drum brakes). Now I just make sure (a) I don't use the parking brake (unless *absolutely* necessary) and (b) try to have the vehicle roll back a bit in park after I take my foot off the brake pedal (I park on a slope). Seems that if the last thing when I park for the night is having the brakes applied, with no free roll in park, with damp weather, the problem re-occurs... Could be your rear drums are sticking.....