Ok, here's a baffler. Unknown year Azuki 10-speed (2x5) 27", with 3-piece crank.

Looking at the crank from the right side, this occurs between 6-9 o'clock, and only when high torque is applied to the pedals. And I feel it through the right pedal only, not the left (despite the torque coming from the left side at that part of the revolution)

BB is solid, no slop that I can make out. FD isn't hitting that I can tell. The front chainrings are not perfectly flat, and do wobble slightly, as does the ring guard, though I can't see anything that they may be hitting, either.

Kickstand was removed first thing, as it was my first suspicion.

So, I'm stumped. Anything embarrassingly obvious come to your minds?