I have the exact same problem, its like a creaking sound almost, mines is only from the front though. I asked about this a few months ago on MW and was told maybe it was too many cheeseburgers LOL......didnt quite get an answer but sometime next week Im planning on checkin it out and seeing whats goin on, I know theres a few things that need fixing
WD-40 is probably good to pin down a squeak if it's a joint or bushing. Try setting the handbrake and pushing up and down on the rear bumper at each corner. Do it again with the handbrake disengaged. If the sound doesn't change, it's probably not handbrake related. Possibly a little chassis creak? Nothing lasts forever.