It's annoying the living crap out of me. Sounds like rear wheel and/or drive train combo, but I can't find what it is. Checked rear derailleur and tightened spokes, nipple looks snug. Tire looks clean on all sides.

Unable to reproduce this noise with stationary bike, upside down, it only happens when I ride it. It seems to produce one click per rear wheel rotation, I was watching brand name on the frontal wheel hub to zero in on frequency.

What the hell can it be? The noise is subtle enough to where it is only audible when I ride next to a wall - that's when I hear reflected noise perfectly well...