Dynamat will basically make the sound waves bounce off of it making your music sound more solid, and less vibrations to get out... but if you car rattles, then there is something loose, etc..
like pSiRacer92 said, you gotta try to hunt down individual parts..

if you have a hard hitting sub, then you'll find out a LOT of stuff rattles.. i know i did when i had my accord..

but dynamat is a good investment... IMO
(if you dont mind the weight :naughty: )