I had a lot of rattling going on in my H3 too. Everytime I stepped on the gas or braked hard, I heard rattling.
I finally decided to look into it a little deeper. I had a Mag Light rolling around on the floor, my jack wasn't fastened in the back door, and a spare ball for the hitch was rolling around in the side compartment opposite the Sub.
Seriously though, I should crawl underneath and make sure everything is tight. After off roading last time, I have heard some banging coming from under the vehicle.