Fan Bearing

The most common, kind of fan bearing is the "sleeve" type. It's a simple design; the steel shaft just sits in a solid bushing. The bushing is likely to be made from sintered bronze, which is porous metal that's impregnated with lubricant at the factory. When that lubricant runs out, the bearing will start to wear, and make a distinctive buzz. The wear will get worse and worse, the buzz louder and louder, and the fan slower and slower, until finally it stops.