Well, I found out where the rattle came from. I mention this, no thanks to anyone else, for the benefit of those with a similar rattle. I can't believe that I'm the only one with this rattle. After a little investigation, I came to find that both "heat shields" that are mounted to the underside (above the exhaust pipe, directly under the rear passenger seat), rattled. These shields seem to be made of aluminum, and when I slightly bent a corner between my thumb and forefinger, the rattle magically became non-existant!!! I'm sure that with the heat of the exhaust, the rattle only became worse, radiating through the floor board and seemingly coming from the "battery vent". Tony