remove the spare tire, car jack and all the tools and do a test.... Check your number plate as well, make sure is secure and not hitting/touching/rubbing the rear of the boot. Check the speakers, the cables/wires/etc. if noise is still coming from the rear, it could be a nut/bolt rolling about somewhere under the carpet or so, remove to check.

Going to look like the commerical where the guy in the holden dismantle his car and found out its the earrings rattling in the coin/ash-tray.