I searched around and couldn't find anything on this, but my 97 Sunfire makes a thumping sound in the rear sometimes when I brake from speeds in excess of 30 or 35 mph or when it has rained. I think it might just be that my drum brakes are bad and that the braking is letting out in the back applying excess force toward the front causing the shocks in back to make that noise as they bounce around. I then decided I don't know that much about cars and better ask you guys first. So anything that you guys can think of throw it out there.