In some countries like the United States, the power grid operates on alternating current with a sixty cycles per second frequency. A few folks, like me, can actually hear the noise in some electronic equipment.
The sound is a low droning hum, which can be annoying for folks who suffer from tinnitis like me. The noise is most prevalent with devices like old-style light bulbs and electric heaters, although some outdoor transmission towers and lines can produce the basso hum.
The noise (and tinnitis) can be blocked by using some form of white noise to mask the sounds. My personal favorite is a ten-hour rain shower and thunderstorm on a tin roof.
Iron Noder 2017