threadend: oooh, oooh, oooh pick me jmlee, I know! I know! pick me, pulleeze pick me
jmlee, after looking about the forum for another volunteer: Yes, threadend?
threadend It's a dry interface between your seatpost and seat tube
jmlee: What makes you believe that's what it is threadend?
threadend: just a hunch jmlee, but I drove myself crazy for 2 weeks once, tightening, tensioning, listening, disassembling / reassembling, feeling my bike, hearing and then not hearing an evasive, rythmic, inconsistant sound just like you are describing. Finally, in desperation I pulled out my seatpost and lubed it. the sound went away.

Unless you have a carbon fibre seatpost, then,uhm, nevermind.