Springtime in the suburbs

Last night our peaceful evening at home was interrupted by a loud humming/buzzing/vibrating sound. What could it be?? We searched the family room, testing all the electrical equipment. It wasn't the tv, cable box, receiver, DVD player, or lamps. We checked the basement. We peered outside into the darkness. I put my ear up to the wall. In exasperation, we opened the sliding glass door and turned on the deck light to inspect the yard. It became immediately clear the offending sound was our neighbor's hot tub. We giggled at ourselves and scurried back into the house.