How to fix a noisy MacBook Pro fan

My three year-old MacBook Pro recently started emitting a constant racket from its left fan (which sounds something like this). Most people will tell you you need a new fan for $50, but I decided to try to fix it instead, and so far (it’s been two weeks nine months), the repair has worked great. As any google or youtube search for “noisy macbook pro fan” produces thousands of results, I hope this saves someone fifty of their hard-earned bucks.