The Culprit

 The mystery noise has been solved!  I had been checking the bottom bracket by pulling on the cranks to see if there was any play.  The bottom bracket felt fine.  This morning I thought I might as well readjust it anyway.  I went to loosen the lock ring and found that it was already very loose.  I tightened it up, and Bob's your uncle, the noise was gone. 

I also decided that I will do the October challenge.  I like the fact it's being conducted via U.S. Mail rather than by Google Docs.  My father worked for the post office after he retired from the military, (before I was born).  He worked the sorting machines and would take me in to the post office occasionally to look at the machinery.  I'm also a card carrying member of the Letter Writer's Alliance.  Solving the noise problem, combined with the way the challenge is being conducted, pushed me over the edge.  Now I just need to measure how far I'm bicycling every morning.

I just checked on the Letter Writer's Alliance web page, since I opened it to get the web address.  Unfortunately, they are no longer active, but the site is up as an archive.