I would apply to the panels since they will tend to resonate easier than the metal of the body. This will help you two fold since you will be helping to keep the outside noise out and you will be adding weight to the panels (which tend to resonate and flex) which will change the resonation point and make it less likely to cause noises. Also....the best stuff to use is Brown Bread. Cheap and great stuff.
--Z is back upand running!
--Who ever said a house is going to be the biggest investment you'll ever make OBVIOUSLY never owned a Z!