Hi. If it is the fan where the power cord connects, that would be the power supply fan. It could be dirty. It could be the bearing is going bad or it is otherwise destined for failure. I would suggest getting some "canned air" (you can get this at Walmart, Office Depot, Best Buy, CompUSA etc). Turn the computer off. Take the side panel off, open it up and check inside. Use the canned air to blow out the fans. While you are at it, check the CPU fan and heat sink. These get dirty as well. If you have a lot of dirt in there, use the canned air in conjunction with a vacuum cleaner. Blow the dirt out of those places you can't vacuum, then suck it up with the vacuum.
The fans inside of the power supply are not intended to be "user serviceable". If cleaning the fans does not fix the problem, you could replace the power supply. I am looking at an ATX 12 V power supply I took out of one of my systems. It has a label on it that says "WARNING hazardous voltages contained within this power suply. Not user serviceable. Return to service center for repair." The unit has two fans on it, one surface mounted with screws easily removable, the other inside and not easily removable. No, being that I am a qualified electronics technician, I wouldn't hesitate to tear into it and investigate a fan that was noisy. However, the warning is something I still take very serioulsy for myself and for you. If you are not capable of doing the repair (if needed) yourself, by the time you paid someone to repair the thing, it would probably be cheaper to replace the power supply in the first place.