Thanks for the assistance on the geiger counter noise.
I'm not sure what the problem turned out to be, and neither is the
LBS. But I brought it in to see if they could find it and they
suggested doing a tune-up and they'd poke around. Then I got a call
that the rear cassette had a bent tooth and needed to be replaced,
along with the chain. I was sort of surprised because the cassette was
pretty new, but something must of hit it at some point. I can't see
how that would cause the noise, which occurred pedaling or not.
But whatever, the noise is gone. I'm not totally thrilled with the
meshing of the new cassette and the old chainring because there is a
roughness in certain gears, but at least I'm not worried about the
bike collapsing under me.