A buzzing sound in a standard wall switch is one of several indicators that the switch may be going bad. Sizzling, popping, or crackling sounds when a wall switch is turned on and off may well mean the switch is defective or worn out.
What's Happening Inside the Switch
When a switch begins to buzz audibly, it means that the current between the metal wire contacts inside the switch is arcing—jumping the gap between metal parts rather than flowing continuously through metal pathways. This causes the crackling sound, and it can lead to burned contacts and heating of the switch and the wiring connected to it—all problems that have the potential for causing a fire. So whenever you hear a buzzing switch, you should view the symptom seriously and take actions to diagnose and correct the problem or replace the switch if it is faulty.
Some dimmer switches have different operating mechanisms, using a variable rheostat in which it is perfectly normal for the switch to make a very slight humming noise when the switch is moderating the electrical current to dim down the light intensity. If a dimmer switch makes this slight humming sound when the light is on, it might indicate a problem, but don't leap to the conclusion that the switch needs replacing.
Watch Now: How to Fix a Hot or Buzzing Dimmer Switch
Diagnosing the Cause
A switch may buzz simply because the screw terminals connecting the circuit wires to the body of the switch are loose. The sound you hear is an audible signal that the electrical current is jumping a gap across metal connections that aren't in firm contact. The first step, then, is to check for loose wires.
Shut off the power to the switch, then carefully remove the cover plate on the switch and examine the screw terminals along the sides of the switch to make sure they are securely fastened down over the circuit wires. You may need to loosen the mounting screws on the switch and slightly pull the switch forward out of the wall box to see the metal screw terminals.
People routinely remove cover plates on a switch without shutting off the power, but this is bad practice, and it has more than once led to a homeowner getting a nasty shock—especially in situations where wire connections are loose. ALWAYS shut off the power to a switch or other electrical fixture whenever examining the wiring.
If the wire connections seem secure, or if the switch continues to buzz after you have tightened the screw terminals, it likely means the switch is faulty and should be replaced. And you see any signs of scorch marks or melted metal on the screw terminals, replace the switch—and soon.