The answer to 'why is my dimmer switch buzzing noise' is that a buzzing or humming sound from a dimmer switch is usually caused by an incompatibility between the switch and the type of light bulb being used. Dimmers work by rapidly turning the power on and off to the light, and this can create an electromagnetic field that causes the dimmer or the light bulb to vibrate and produce a buzzing noise.
Some common causes of dimmer switch buzzing include using the wrong type of light bulb (incandescent bulbs work best), having an old or faulty dimmer switch, or having the dimmer switch installed incorrectly. The buzzing is usually harmless, but it can be annoying. To fix it, you may need to try a different type of light bulb, replace the dimmer switch, or have an electrician properly install a new dimmer.