Mine creaks about a day after I have recranked all the seatpost QR's. In my case there's fretting going on between the 2 frame members.
Cure: from a yoghurt plastic container top, cut a disk with an ID of 34, and an OD of 42 or so. Just slide it over the seatpost between the frame halves.
Mine still creaks a bit, but much less often. Retightening the QR's seems to stop it again, at least for another week.