As others said, most likely the doorbell transformer. But possibly instead a transformer for powering a furnace humidifier (ugh!), or sometimes even an alarm system.

But most likely, for the doorbell(s).

As for 60Hz hum, the transformer itself will typically have an audible hum. So if that is audible in the recording room, then it might be picked up there.

But 60Hz electrical noise is also present in all of the home wiring, including the wiring powering the audio gear. And the most likely source of 60Hz in a recording is from the 60Hz electrical noise of that wiring.