I had a similar problem. I found it to be the rear-passenger seat-back release/adjustment cable. There is a latch next to the headrest for each rear passanger seat. This latch is used to fold down the seat back, or to adjust the angle of the seat back for each passanger.
The release/adjustment is done with a cable that runs down and under the seat to the outside rail of the seat. If you look under the seat you will see a black shrouded cable coming from the center-side of the seat to the outisde rail. This shrouded cable rattles inside the plastic molding of the seat.
I found that if you pull the shrouded cable back to the rear of the seat and secure it with a cable tie to the large wire at the rear of the seat, it will put enough traction on the cable inside the plastic molding to eliminate the rattle. There are several wires making up the support of the rear seats. You want to use the cable tie on the larger wire going left-to-right at the rear of the seat bottom.
My RX350 has never been so quiet since I put a cable tie on these two realease cables.
