If it's still under the 36k warranty the dealer can take care of it. I'd start by looking at the trim around the pillars. I had a few squeaks in mine around the pillars that the dealer fixed way back when. They are tough to locate while you are driving, best thing would be to sit in the back while someone else drives so you can really look around. Find a nice bumpy gravel road/drive and start pressing on stuff to see if it stops.
Cupeez; LED Interior + License Plate Lights; Curt T Wiring Harness; Torklift EcoHitch; Weather Tech Floor Mats + Rain Deflectors; Kicker Speaker Upgrade