Muffler rattle + sub = not good?

[JDogg]

My 12" sub is rattling the hell outta the rear end of my Levin (mainly exterior, particularly the exhaust), since searching forums Ive deemed that a check of all nuts/bolts is on the menu first up. But my question is this: the muffler has been getting worse over the past week or so (maybe longer, only really noticed it tonight)... if its not loose nuts then could it be internals of the muffler going/gone? Anyone have similar experiences?

Am keen to get it quietened down, the sounds are all good on the inside, but the rattles outside are starting to get me funny looks!

[bridgeport]

put styrafome behind the numberplate take the tin heat sheiild off thats above the muffler and attached to the car couple of 10 mm bolts

[JDogg]

Cheers for that.

Im gonna look at removing my tail light array and sound-proofing the light fittings if possible... Got the car up on a drive-on hoist and found out that the underside of the car isn't my problem here, it seems to be more the light fittings/interior bodywork.