I've isolated it to the bottom bracket:
- The noise also happens when I'm standing, so it's not a saddle creak.
- The noise is present if I spin with only one foot, either side; so it's not a pedal problem.
- I checked the crank bolts and the arms are not loose.
- I checked the chainring bolts and they are not loose.

It happens at the same spot in the pedal stroke, forward or backward. It's a click/tick as the crank moves past a certain position. It doesn't happen without pressure on the pedals; just spinning them in the stand won't do it, but if I apply the rear brake and use some force on the cranks I can make it happen.
The BB doesn't feel as though it's sticking at that spot when I spin it by hand. It's a less than 1yr old UN54, so I don't think it's a problem with the bearings.

I'm going to take it out and regrease the threads and see if that helps.

The Question: Would riding all fall/winter/spring through the soaking rain eventually deteriorate or wash away the grease I used when origainally installing the part, leading to this noise?