I'm a sucker for these threads. I've got no less than 12 bikes, and I ride all of them-just not at the same time. Anyway, with that many bikes, that's a lot of possibilities for some creaks, squeaks, clicks, and ticks, and believe me, I had every issue imaginable. I don't know the solution to the OP's problem, but whatever it is, it is probably something you didn't expect.
Noises have a way of sounding like they are coming from a certain place and they wind up coming from another. I always stand up when I hear a noise- to see if it's coming from the seat/seatpost. Then I move on from there. A friend once suggested I stop and check my quick releases. I thought to myself, "he must have lost his mind." To appease him, I did just that and the creaking stopped. Turns out, I was the fool. Good luck in tracking it down.