If it's a fairly new bike and the clicking happens once or twice per crank revolution, there's a good chance that it's simply the chain passing over the shift gate on the outer ring.

The shift gates are the cut down or missing teeth on the outer or middle rings (innermost rings don't have them). As the chain passes over them it can move over slightly then catch on the next tooth. It's comparable to the sound a train wheel makes going over a switch.

You can confirm this easily enough, by noting the crank position when it clicks, and then putting the crank in the same position and seeing if the gate is in the 11 o'clock position just about engaging the chain.