It happens while coasting too, so it's not you or your shorts, right? The rhythm is related to tire rotation speed, not crank speed, right? If you were riding sewups, I'd say it could be a loose rimstrip edge touching the brake pad (right kind of noise). Maybe it's something silly like a leaf stuck on the bike somewhere, or a glob of something sticky on the tire/road surface. I'm liking the sticky tire tread theory - that would give no sound on the stand. I had a similar sound this weekend (but more chunk-chunk sounding). Put on a new tire and got a noise from the front wheel. It went away after about 10 miles and I decided it was probably a spot of glue or something on the tire tread. But I still get it if I'm pedaling standing up, because the bike is swinging side-to-side more than if I'm seated so I'm riding on a wider amount of tread. It will wear off eventually.