Horrendous rattle inside cab.....

ok, I have an 08' X-runner still N/a for the time being, and unfortunately when I accelerate aggressively in first and early second, I get a gnarly rattle that sounds like something is just loose inside the cab behind my seats. Ive tried buckling the seatbelts, taking the jack and tireiron out, stuffed towels inside the little cargo boxes, and still I get it. Has anyone else had the issue, what is it, where was it? How did you fix it? This is just aggravating the crap out of me and I dont think I cant take it to the dealer and say "hey I have a rattle when I accelerate in low gears hard." or can I? Thanks guys!

 

Update?