I have had a similar sound, a horrible vibration/rattle coming from below my floorboard. It was caused from my installation of a Tenzo R short shift kit. It was rubbing too much against my heat shield, which at certain speeds caused varing noises of misery. I could have removed my heat shield, which ran the length of my floorboard but that would have caused excessive heat in the passenger compartment. So, I cut out the offending section and replaced it with a fiber like racing wrap, taping it in place with an aluminum heat proof tape. Sound gone, no heat transfer....I also replaced my cat with a Random Technologies free flow cat and checked my exhaust pipe connections and hangers.