The tormenting buzz

Some electronic demon in my home office has taken to buzzing at apparently random times. (I say "apparently" because I believe the randomness is an artifact of my lack of understanding of what's going on.) You might think such a demon would be easy to kill: go to the source of the noise, pick up the offending device, and punish it appropriately (perhaps by a temporary servicing, perhaps by tossing it on the junk heap).

Not in my office.

I know the source of the noise is somewhere on the left side of the U of Power. The problem is, that area of the office contains three computers, a monitor, a NAS device, a cable modem, a wireless router, a gigabit switch, a few power strips, at least two UPSes, a printer, and a copier. The wily demon is also quick--before I can get close enough to locate it, the noise stops.

I expect much cursing ahead.