The fan on my MacBook had been dying for almost a year. It spins with a strange grinding noise sometimes. Then it will go back to normal again for a while. Recently, I finally ran out of patient. The noise was simply unbearable.
Hunted down a replacement fan online, took out the old fan and popped in the new one. My MacBook is now anew. Not only it's much quitier, it's also running cooler now.