This is possibly the most frustrating thing to find on a bike. I thought that mine was BB, replaced it repeatedly, had the threads chased and the shell refaced, still there. Thought it was pedals, lubed them up as they were actually squeaking a bit, lube stopped the pedal squeak but the BB creak was still there. Checked the front end for tightness and sound, nada. Finally got another wheel to run and the sound's gone. Rear wheel tension was off, problems were plenty. I actually rebuilt the wheel only to have an eyelet give out on a non drive side spoke where the tension was fine. New rims arrived today, LBS is building them up for me this time. Check everything in the drivetrain, lube the spoke crossings of the wheel, check the rear hub for tightness, clean up the rear dropout and hub really well. Basically, it could be anywhere on the bike. The only thing you can maybe rule out is the saddle creaking since it does it when you're out of the saddle. Just be patient, and look at it as a potential excuse to buy new stuff!