Problem is solved; the hum came back this morning right at 9:00 am. By chance, the wife walked into a far area of the back yard and heard the hum. It was coming near a block wall on the rear of the property. I looked over the block wall and discovered the source. It was from a neighbor's pool pump. Not the neighbor's pool directly behind us; his pump was off. It was the neighbor NEXT DOOR TO HIM, and not directly behind us! Same sounds, same frequency.
Thanks for all the suggestions from those who offered help.
