For a bit over a week now, my bicycle has made a distinct "tick" sound almost everytime the crank turns - at the same rotation each time (when my right pedal is at about 5 o'clock). The tick occurs when I pedal with both legs - peddling with either the right or the left leg exclusively I don't get the sound, so whatever is causing the noise must be related to alternating stresses on the crank machinery. Same sound clipped in or with platform pedals. I can't reproduce the sound when I'm not actually bicycling.
Any ideas what might be causing the sound, and what to do about it?