Why do lightbulbs 'hum' before breaking?

[Henryn]

I always know when a light bulb is about to go because it starts humming, but why does it do this?

[Isaacr]

When you hear this sound, the filament has already broken, but the circuit, and therefore the light emitted, is maintained by an arc of electric current between the two broken ends. Arcing like this always produces a sound, like the hum of a neon light, and the pitch is determined by the resonance of the light bulb.