I took my 13" rMBP into the Apple store for the same issue. The genius tried re-torquing the screws for the bottom case and in the process scratched the bottom case. He installed a new bottom case for free, which corrected the creaking issue by the lower left palm rest area. I then asked them to check into an issue I was having with the display sleep timer not functioning correctly. They sent it out to be looked at and in turn scratched the black part of the hinge cover. As a consolation, they gave me a brand new unit as a show of "good faith". 😀 They'll look at it for free and correct the issue for you. Good luck!