It's probably the bearings inside the 3-piece cranks. Get some Tri-Flow and pull your crank arms off, then put some tri-flow on the bearings. This will likely get rid of the noise you are hearing. It is also possibly from the cranks (not as likely) or the pedals or somewhere else. You really, REALLY want to try to isolate exactly where the noise is coming from. Loosen your chain if it is tight and see if it continues. You can spray you chain with Tri-Flow, but then you will need to degrease it (gasoline or other) later on as that is not a good move.

I would start with the bearings in your bottom bracket and go from there.