The cooling fan is making annoying sound

Hello,
I have a desktop pc where I work on over 15 hours a day. So I really have to pass a lot of time around it. Recently it is making some annoying sound which I think is caused by the cooling fan inside near the power supply. I have tried to figure out what happened but could not do so. Please give me some suggestion on how to make this annoying sound stop.
Karlos Hilton
