Hey guys, I finally got my system set up except for a good amp in the 500E. I put a JL 12W3 with a custom built box in the trunk. I did not port the sound in, because the sub is so loud already that I have to turn it down because it hurts my ears

But anyways, the problem is my trunk rattles like crazy! And i cannot find what is ratteling. It's not my license plate, and it's not the bumper because i've tried holding the bumper still, and it makes no difference at all. It is something underneath behind the bumper. Anyone have this problem and know what is ratteling?