power hum

Power hum is the low-frequency buzzing sound produced by electrical equipment operating on alternating current, typically at 50 or 60 hertz depending on the country's power grid.

It's the drone you hear from fluorescent lights, amplifiers, or transformers: There was a faint power hum coming from the old refrigerator.

In audio recording, power hum is unwanted interference that contaminates a signal, often caused by poor grounding or electromagnetic fields from nearby wiring.