this is a mechanical issue.
see if your fan is serviceable. If it has a sticker or rubber cover to the bearing, you can try putting a drop of lubricant oil in there. (not wd-40, wd-40 is not a lubricant)
article with pictures:http://www.techrepublic.com/article/quiet-noisy-computer-fans-with-a-drop-of-oil/5034842
You can also just check to see if there are any loose wires or even dustbunnies, hit it with a vaccuum cleaner to see if that sucks anything out.