Why does my fan sounds like it's off track?

I've had my PS3 slim for two years now and it works perfectly. I can play all my games, Blu-Rays, digital media, and everything else just fine. However, a year after buying it, the fan inside makes a rattling noise. The console works like it should but the noise throws me off a little bit. If I firmly tap the console one or more times, the rattling stops and the fan works perfectly again but when I want to place the console on its side on a stand, the rattling starts again. This is an annoyance because I want sell my console and games to someone else so I can upgrade to the PS4 and I wanna make sure I can get as much as I possibly can with a fully functional console with no problems. If I have to buy another fan, I will but if there is a fix, please let me know.