Sometimes when an incandescent light bulb is just about to burn out, the tungsten filament begins ringing.
What causes this?
For people who unfamilar with this effect, what happens is that every once in a while, when a bulb is near the end of its life, it begins ringing or sometimes buzzing. The ringing lasts typically for several hours before the bulb finally burns out, but sometimes can last for days. It only happens in a relatively small percentage of bulbs, maybe 1 in 15 or something like that.