Some changes from the defaults that make the Frequency analysis window a bit more useful
Ah, got it, now I see the 120Hz spike.
Ironically the 120Hz noise is about 4db lower in the middle of your three samples, indicating that moving the mic over next to the monitor moved it away from the source.
Actually, the middle sample is one in my normal recording position. The only difference between sample 1 and 2 is mic position (monitor is on in both 1 and 2) - indicating that normal mic position is better than close to the monitor. But the only difference between sample 2 and 3 is the monitor state (plugged in in 2, not plugged in in 3) - indicating that unplugging the monitor made things worse... ?
The light is a single tungsten bulb in the closet I'm recording in. There are more tungsten bulbs in the bedroom about 10 feet away behind a closed door.
I did the experiment Koz mentioned, recording and saying what position I was in. I then went and plotted the spectra for six different positions:
First test: mic close to monitor, 120Hz level pretty low --> probably not the monitor
Second test: mic close to the light, 120Hz level 9dB higher --> it's the light!
Third test: mic close to the light, light switched off, 120Hz level the same --> it's not the light?
Fourth test: mic close to the light, light switched off, closet door closed, 120Hz level 5dB lower --> ???
Fifth test: mic close to the closet wall that has the PC on the other side, 120Hz level low --> it's not the PC
Sixth test: open the door, put the mic directly next to the PC, 120Hz level very high --> the PC is loud, but not getting through the wall.
My conclusion from this is mainly that I'm confused. Could it be the light fixture itself, humming even when the light is switched off? Or could it be power to the microphone from the PC (i.e. an electrical hum, not a recorded sound?).