i noticed a rattling noise coming from the back of my car, it happened at low speeds over bumps. it sounded like a metal on metal vibration, after poking around underneath my car i eneded up being the metal shields right behind my rear rotors. they were just bent out of shape.
if you have the same problem, then you should check this, jack up you rear end and smack your tires from underneath, with some force. if it rattles then you just need to bend the sheilds untill it doesnt rattle anymore.
i know someone else had this problem...