Last week’s column featured letters from readers about the possible causes of mysterious household humming noises. Here are more readers’ thoughts for a homeowner investigating such a noise:

• Wayne Collins: “It is called ‘sympathetic vibration.’ Some part of the house, perhaps the A/C ductwork, a wall, or window, has a natural frequency that matches an energy source some distance … away. There may be a factory, generating station, or pumping station that runs all the time. Something in the house is vibrating in sympathy with that machine. It has a resonant frequency that is equal to, or a harmonic of, that machine.

Perhaps, by going around the house with a stethoscope, and by pressing on the air ducts, windows, walls, you can find the ‘speaker.’ Then, by adding some dead rubber or a stiffener of some sort, you can eliminate the hum.”

• Thomas Burgun: “I had that once. It was my sump pump, with a bad float switch happily burning itself up.”

• Lee Galiato: “I can’t help wondering if the homeowner has a ‘solar-powered’ attic fan with ‘battery backup’ … on the roof or under a gable on one side of the house, which could explain … the humming even after tripping the main breaker.”

• Hank Graham: “Some septic systems have built-in aerators that are basically small propellers that stir the wastewater in the tank. Get a stethoscope and listen through the walls, floors, and ceilings to at least try to isolate the source.”

— Contact the writer: aheavensphillynews.com or at The Inquirer, Box 8263, Philadelphia PA 19101. Volume prohibits individual replies.