Noisey rear end...rattling brakes i think ? help

well lads,,need a bit of help here...im getting a loud rattle from the rear end whn goin over a bumpy surface,.at first i thought mabye rear arb bushes..but today i noticed the rattling goes away when i press the brak pedal and go over the same bump..i tried it a few times justt to be sure and there is definatly no rattle when applying the brakes.. but without applying the brakes,,,its very loud ??