Katydids or Buzzing Brake??

[ndbigfish]

I went for a little ride this evening at dusk. I noticed that I have a "buzz" somewhere near the rear brake. I recently changed the rear pad and only have about fifty miles on the rear pad; most of it in town. The noise is there regardless of being under use or free wheeling. The tone of the noise changes when the rear brake is applied, but returns upon release. It does not increase in frequency at higher RPM. The sound is similar to a katydid; just a constant buzz. The anti-rattle clip seemed in good order. Any thoughts on this one?