I'm looking to possibly insulate the interior of my car(door panels, truck panels and under floor, hatch panels) to keep the noise level down (not expecting to make it dead quiet) and to keep things from rattling due to road vibrations and what some would call an overpowered subwoofer any suggestions for what to use? The exhaust resonation from the hatch is one of the main things I'm trying to reduce

You could use a two part expandable foam, but you have to be careful and take precautions to not let it go where you don't want it to. It can be lighter than most insulation materials and will not only reduce sound levels, but will also help stop things from rattling in the first place. The only bad thing is that it is pretty permanent and you wouldn't want to use in areas that you have to get to often.