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