My '05 X-Runner had this annoying rattle coming from the passenger seat. I pulled off the passenger seat side cover and found a bracket loose and rattling... It looks like a design flaw as there is no way to really tighten it. It is the 'L' bracket that controls the seat slide when the seat back is placed forward. I could duplicate it by smacking the top of the passenger seat and watch the bracket rattle away.
What I did to fix it was to put some electrical tape over the elbow joint,just a few raps, enough to stop part of the rattle and but not too much to stop proper operation. Then I installed a large (3") rubber fender washer under the bracket where it attaches to the seat on top.
This only took a few seconds and my rattle is gone, and the seat still operates properly....
I was going to take it to the dealer, but last time I was there 3+ hours, came back with the same problems and 2 extra door dings... talk about service...
(sorry no pictures)