Went riding tonight and my seat was creaking. Go figure. While we were checking out the bike trying to locate the creak, the two guys (employees at the LBS) both told me I needed clipless shoes and pedals. I expressed my reservations about falling over, and they told me to harden up
Try taking the seat post out, clean it and the tube, put some grease on the seat post (I'm partial to trailer bearing grease) and re-install the seat post. Good luck. Bike noises can be really annoying.