I've been following this thread and reading some replies by some very smart and knowledgable people. I thought maybe we might be able to use some thoughts on the issue from someone not so smart and not so technically knowledgable (me).
If one were using Audacity I would recommend first trying the repair tool. If the noise is too many samples long in duration for that, then click repair sometimes works (there are adjustments to how sensitive and specific the tool is -- that can have an effect on what type of click is removed), or even splitting the channels, highlighting the noise, and using negative numbers with the amplify tool until it resembles the baseline, then attaching the channels back to a stereo track.
In terms of "what's the cause" -- there are countless causes of noise, as people have pointed out. Loose cables, RF interference, loose wire in the mic, a power surge from the battery/power source, and I also sometimes wonder about static electrical discharges when touching the gear to make adjustments.
Good luck. Sometimes I record a show and there are a dozen or so UNOs (Unidentified Noisy Object) and other times there are none. i wish i could see a pattern. Maybe someday. Thanks to all the people who contributed to this thread -- i continue to learn quite a bit on TS and I really do appreciate your time and effort.