Hi L.
Long time no post.
If you can change the sound you can at least narrow down where it might be.
Is the rattle different if someone is sitting in the seat?
How about moving the front passenger seat to a different position?
How about folding down the rear passenger seat?
Could the rear rattle be the tailgate? I had a rattle in the E53 that I solved by adjusting the rubber stops on the gate.