OK, well, what usually happens is that the bushing goes bad, or the shaft where it goes through the bushing. The lube fails and the metal starts to wear. You can see the shaft looks a bit burnt if you take the motor apart (which I have done several times in another life). You would think that a dab of Kroil would set it to rights, but, no---only temporarily, if that. The bushing hole gets out of round, or the shaft gets rough, and that's the ball game. Noise is sometimes caused by vibration of the windings in the rotor when the lacquer quits because it's old or badly applied, or poorly formulated in a country where they think it's a good idea to put poison in baby food. A wobbly fan blade assembly can get to oscillating, slapping the rotor to the end of its travel. They are a few of the problems I've seen with small motors and fans. Hope that helps.