Replaced both fans in my MacBook Pro
One day my MacBook Pro started rattling very loudly whenever the fans came on. It was very annoying and got in the way of my work.
It took me about a half hour to complete the repair, except that I had to run out to get a new screwdriver in the middle of the repair. The repair described in the manuals said I would need a phillips screwdriver. But the screws that held down the fans in my MacBook Pro were actually a very small Torx screw (a star hole in the head). I had to get one since it was a very unusual screwdriver to have. Once I had that, the repair went very smoothly after that.
Check to make sure which screws are actually holding down the fans - Torx or Phillips and get the right screwdriver.
Phillips #00 Screwdriver
MacBook Pro 17" Unibody (Mid 2010-Late 2011) Left Fan
MacBook Pro 17" Unibody (Mid 2010-Late 2011) Right Fan