Be 100% sure it's nothing in the spare tire well. A loose tool, perhaps even a loose spare. Also one thing I noticed in the past, I have a speaker box pushed up against the back seat, and i used to shove the car owners manual between it and the carpeted bottom of the rear window shelf, so there was pressure pushing up from the box into that inside bottom cover. Used to make all kinds of creaking noises over soft bumps...removing the jammed in manual got rid of that sound, but I thought it was something much more complicated until I figured that out. All I'm saying, be 100% sure what you are hearing isn't something other than suspension.