The hum in C is starting to feel like a conversation

I've been chasing a ground fault in a panel down at the old mill, and the 60‑cycle hum from the main breaker has settled into a perfect C. It's not a problem—I checked everything twice—but lately I catch myself listening to it the way you'd listen to a person breathe in the next room. I'm not saying the wires are talking to me, but I'm not not saying it either. The night shift does things to a man's head, I guess.