Jim we've been using sound deadening mat from Noico, its 80mils thick, foil faced with self adhesive bitumen tar like, you have to burnish it into place with a piece of smooth wood or heavy metal roller, once in place it really does a nice job..its a bit cheaper than Dynamat but from what we've seen just as good..Did all the cab body panels in my Chevy K30,inside the doors,etc,really helps keep it as quiet in the cab as a 350 with headers can be. The doors when shut felt tinny and light, man put the Noico on the insides of the doors WOW now they shut right and feel really solid.
We've used the GTMat brand as well, the Noico has been the best..on eBay all day long,check Amazon as well.. FWIW,just our experiences with it
