Hmmm... lots of interesting ideas here.
Pokey: I have checked the bike in a workstand (which means here a piece of rope over a beam). I *cannot* reproduce the sound no matter what I do: shifting to all gears, holding the back brake in various positions, even pushing on the fenders a bit. The fender clearance at the brake is narrow (about .5 cm), but the narrowest point is the center of the wheel. The clearance is almost 1 cm on the sides.
I don't think it is in the bottom bracket (which is sealed, BTW). I just scrubbed down my chainrings and the BB and cranks all seemed in order (yes, the sound was there before this maintenance). I suppose I could pull the cranks off and check whether I feel any scraping. Also, as I said, the sound occurs in rhythm with the turning of the wheel--although I have to be pedaling to hear it.
I will also inspect the headset, and the seatpost. Those are both odd, but not infinitessimally small possibilities.
Cracked frame? Geez, I shiver at the thought. But, to figure that out, should I do more than a visual inspection? BTW, this is a steel bike, so the typical Al cracks are not very likely.
I suppose that the next best thing that I could do, is to take the fenders off and see if I still get the sound. That would rule out the scraping possibility.
Any other thoughts?
Thanks again,
Jamie