what kinda hangers are holding up the exhaust in the middle if any? you need to make sure you have that middle hanger holding up the middle of the pipe, and make sure that it has the rubber hangers on it, i used to have this problem when i first put my exhaust on my car, everytime i rev up and let it go down, it souded like there where some beads in my exaust ratteling around. another time was i had washers on my down pipe bolting it to the catback, the washers were loose and they rattled, check both of these and let us know what you find out.