There are so many possible causes of various sounds that it's not productive to blindly compile a list of possibilities (ex: "Hey that happened to me once and it was...") What you need to do is to observe and then convey to us the following:
(Note to others with problems: The list below is applicable to a host of issues, so keep it in mind when posting a request for help)
- What is the bike model, and how old is it, purchase new or used?
- When did the problem start, and did any kind of incident, equipment change, or repair procedure immediately precede it starting?
- Does it occur in certain gear combinations (front/rear) or any combo?
- Does it occur under light/heavy pedal pressure or both?
- When it occurs are the cranks (specify right/left) in a certain position?
- Have you tried anything to correct it or eliminate a possible cause, and what did you do?
- Normally I would ask about the frequency - in time with pedals, wheels, every x pedal strokes, or random, but you seem to be saying that it happens either about once per pedal rev or twice - is that correct?
p.s. Previous experience leads me to request that you answer all the questions, not just the ones you think apply. If you are not sure, that's a perfectly acceptable answer. Thank you.