There's several things it could be. Most common culprits are the rear sway bar bushings and the parking brake cables rubbing on something. I had a really annoying creaking noise a couple years ago that I couldn't find the cause of. I asked the shop to look for it while I was getting my oil changed and they found it, it turned out to be a metal cover that protects the wires to the RDU clutch. It was rubbing on a large washer thats part of the RDU mount. The mechanic just reached up and bent it by hand so that it wouldn't rub anymore.
But it could be any bushing or any part that moves.