i have been working on it for a few days and finally figured it out.

first, it must be said, the seat and it's assembly are pure crap. i can't believe toyota would even bother using such poor quality seats in their vehicles. ..anyway, since they did, i figured it was up to me to fix the problem.

after much diagnostic work, i realized that the channel that the seat slides in is not manufactured as precise as it should be. ..it's at least 1/16'' wider at the back of the channel than the front. ..my solution was to take the seat out and put several C-clamps on the channel at the rear and tighten them down. ...i think i ended up tightening it a little too much because it doesn't slide as easy now. ...but the most important thing is that the rattle is gone.

...just thought i'd share this info since i know my seat can't be the only one making the noise.