I removed my back seat and applied some stuff from Lowe's to the back wall of the cab. It's a black mastic looking roll with a foil backing, I think I found it in the roofing section. applied two layers, did wonders for the sound level in the truck. The back wall is simply thin sheet metal and drones like a drum when you tap on it. (I don't have a muffler) I would imagine, if I had time to remove the seats and carpet and apply it to the floor of the truck (and firewall) the truck would be like a tomb. The stuff did gas off, but it was very minimal.