actually, if your bumper is rattling and not your back hatch or deck lid, all the deadening in the world won't completely stop it. what kinda car is it? are the body panels made of metal or fiberglass? have you had a system in there rattling **** loose for a while now? without knowing this info, i'd venture to guess that you'll need to install some thick rubber grommets and silicone anywhere where the panels touch the frame or other panels, and where the bumper bolts on. Might even have to stuff the offending panels with styrofoam peanuts or something like that.
if it's just the hatch/deck lid and liscense plate rattling (which is the case in about 90% of installs) and not the panel itself, thick weather stripping and sound deadening mat/spray will probably cure it.
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