I have a 12" 350W RMS in sealed enclosure in the trunk well. Whenever I crank up the volume, the trunk/rear bumper started rattling. Trunk already deadened and license plate padded. What could be causing this noise? It seems to be coming from inside the rear bumper... I know some of you are running multiple/high power sub in the trunk, what did you do to stop the rattling?

 

Have you checked the underside of the trunk well? It's the big bowl shaped thing between your exahust. Not sure if it would rattle but there may be something near it that is rattling against it.