just to make sure it is definitely the power supply:
unscrew it from the bottom of your PC, and use a small object such as a paper-clip to hold your PSU fan while starting up the power supply just for a few quick seconds. if there is no noise, then it is definitely the fan. FYI it is safer to do this with your PSU out of your PC rather than connected to all your components. there are lots of guides online, here's a quick one I found via google:
http://www.instructables.com/id/How-to-power-up-an-ATX-Power-Supply-without-a-PC/
if you're sure it is your PSU, you can try to fix it yourself just by visually inspecting the fan and making sure it's not hitting anything. if not, then it's possible that the fan is going, in which case you can unscrew the fan cover and replace it from there, since most PSUs use a standard fan size such as 120mm. alternatively, if it's still within warranty you can always RMA it to the manufacturer.