John just bought a powered subwoofer for his home theater and when he plugs it in, it makes a humming noise. Leo says that's called a "ground loop" and there's ways to fix that. The plug isn't properly grounded, or could be loose. Or the stereo itself is causing it. John should go to RadioShack and get a magnetic ferrite coil that snaps onto the wire. That will eliminate it. If it doesn't, then an electrician may need to check his wiring and power.