It went away after a couple of hours... or maybe I just tuned it out! It came back during the day today for about 30 minutes, and stopped.
I think it's a septic pump alarm on a house that's currently not occupied (owners spend the winter in Arizona) and it stops when the tank has "gravity drained" below the alarm trigger... then the caretaker comes in and washes something, or flushes the toilet... and it goes off again.
That's just a guess, though.
That's an interesting possibility. I hope it doesn't continue all winter.