Given that you have indicated that your chain and drivetrain is well lubed, no apparent problems with pedals, and that this occurs at the same point on every rotation of the cranks, I would suggest it may be linked to either:
- Cranks
- Bottom-bracket
- Pedal bearings
First try spinning the pedals on the axis of their spindle to ensure the spin smoothly and approximately equal on both pedals.
Next check the cranks. Try pulling each crank gently outward to see if it has any play or movement. If it does it might simply need to be tightened with a hex torque wrench (to the manufacturer's recommended torque).
If it is not the cranks, it could be something with the bottom bracket. Unless you are skilled bike mechanic, I'd recommend going to a pro to look at the bottom bracket.
This site might have some useful tips for troubleshooting the sound:
Diagnosing and curing strange noises from your bicycle.
sheldonbrown.com