Rattle sound in middle of car beside drivers seat?

I had the exact same problem with my 2005 MDX. The sound comes from the long, thin metal bar that runs the length of the seat as part of the seats frame. I discovered that I could stop the rattle by reaching back from the driver's seat and grabbing the bar and putting slight pressure on it. I solved the rattle by cutting a small wooden block and wedged it, vertically, under the bar (between the floor and the bar) right in the middle of the length of the bar. It's been working for over 5 years now.