I'm going to weigh in with Hillrider and Pete Hammer (always good company to side with by the way). I doubt your crankset is the problem. If it were, it would likely happen at the same place of the pedal stroke. Bike noises are goofy. They all seem to sound like they originate at your crank.
Actually, my first bet would be your chain. Try turning the cranks backward with your hand and watching the chain when it comes out of the rear derailleur cage. My bet is that you'll find a place where the chain has a tight link.