I added Dynamat Xtreme liner to the underside of the lift bed for rattles, and dyna hood liner/sound deadener on top of that everywhere the engine is exposed to. Seemed to help quite a bit. A lot like when I added the hood liner to the engine cover of my ski boat. Helps reduce the drone of the engine.
I'll probably add more of the hoodliner to any other areas near the engine that I can get to, maybe behind the plastic kick-panels in the rear near the engine.