Sorry if this sounds stupid, but I've done it myself. Look down when you pedal, and check that you're not just hitting the end of the cable from the front derailleur with your foot as you pedal. That will cause a ticking or pinging sound with every revolution. And to make it more confusing, it can occur only in certain gears.

Otherwise I would check first of all that there's no play in the pedal arm, which could indicate a bad bearing - hold the pedal arm parallell to the lower horisontal bar of the frame, hold them both firmly in one hand and squeeze together. You should not feel any movement.