What it wasn't:
- seatpost or seat;
If it was so, there'd be clicking even as I coasted over bumps.
- chainrings, bolts seemed tight
- bearings; there was no "play" in the crank
- crankarms; bolts were tight
- chain. This was a major suspect, as I'd botched up installation. But it wasn't, as I'd lubed it, and the links weren't stiff.
What ut was ... the right pedal. There are reasons why it couldn't have been, so it was the last thing I checked. As Sherlock Holmes said, "when you have eliminated the impossible, whatever remains, however improbable, must be the truth".