Umfan, take out the rear seat cushion (simple) and tighten down the four bolts holding on the back of the seat. One might have worked loose causing your noise. Good place to start at any rate.

Besides the odd pop or whatever, the only problem I have with noise is the shifter handle. It makes a lot of squeaking when moving it back and forward, but I haven't gotten around to taking the boot off so I can lube it.