the rattles you are hearing is caused by one or two of two things. the sheet metal flexing, or two things banging against each other. before you go paying $80 for a dynamat trunk kit, try to isolate the sound. it could be the lic plate hitting the bumper, or something just as simple.

the thin sheet metal on hondas WILL flex and make noise. to prevent this, you have to add weight onto it. that is one thing dynamat or other sound deadeners can do. dynamat is only one brand of sound deadener in a big market. if you do a search in this forum, you will find a whole variety or cheaper and "better" alternatives.

the "foam" stuff is also a form of sound deadener that comes in a can. i believe there are two kinds. one that is similar to spray on cement (not as heavy of course) and another that sprays on, then expands.