You don't pick up the hum from being plugged into the mains. Nearby equipment generates sound at the frequency (and harmonics) which are picked up by the microp

You don't pick up the hum from being plugged into the mains. Nearby equipment generates sound at the frequency (and harmonics) which are picked up by the microphone. Various other equipment is sensitive to the EMF. As others mentioned its pervasive indoors and near power lines.