I'm a bit stumped by this...
My rear is clicking (ahem) once per revolution, and the odd part is that the click sounds like it's moving from side to side. That is, one click will sound like it's coming from the left side, the next sounds like it's in the middle, then the one after is definitely coming from the right side. Then it moves back the other way.
It doesn't do it in the stand - only when riding (like all annoying noises).
Perhaps my advancing years have finally caught up with my ears / brain.
I've checked the usual stuff - bearings and freehub seem fine, cassette lockring's on properly, brake disc is straight and not rubbing, nothing flapping on the wheel / tyre, no loose spokes.