Loud PSU FAN

Solution
Replace the fan if you know how to do it without hurting yourself or bring it to a tech who'll do it for you. It's worth it only if it's a quality PSU that no longer is under warranty. If it's a crappy PSU, then the recycle bin is the best solution.
There is enough power in a PSU to kill you if you don't know what you are doing. From the nature of your question that would appear to be the case.

If you can't see anything rubbing the fan to make the noise from outside the PSU case take it to a shop and have them open the PSU case and explain to you what the problem is. Then you'll know next time.

Here's a sample of what you are dealing with when you open the PSU case. Notice it only took one short moment of inattention to end in disaster
http://www.dailymotion.com/video/xfyyii_man-electrocuted-on-the-train-india_news
 
Enough to get a serious shock, but it doesn't kill people.