I have SPD pedals, and one of them clicks as I reach the top of the pedal stroke (crank arm reaching 12 o'clock). I know which pedal it is because it doesn't click if I unclip and pedal one-legged. It clicks when I'm clipped in, and it still clicks if I unclip and just put my foot on top of the pedal.
I checked the pedal and it doesn't seem to be loose or anything. What could be making the noise?