Well I tried increasing microphone gain and volume and computer volume and playing back through the computer. It was still hard to locate any change of hum inside the closet when I moved the mic around, and nothing changed when I touched the mic. I did notice a slight change when I got down close to where my desktop sat on the other side of the wall, though, so I decided to drill a hole in the wall so that cord length would allow me to put the desktop about 4 feet away, and that seems to have solved it! When I plot the spectrum of noise, I rarely have any 120 hz peaks, and when I do they never seem to be higher than -70dB or so.
I also decided to re-record, being more careful about maintaining mic position between recordings. Here's the (raw) transition between a recording I finished last night and one I started this morning. The difference seems much less pronounced to me.
Any other thoughts on the sound quality of this sample? Does it sound like decently clean raw audio?