Typically noisy computer fans are caused by the bearings inside the fan beginning to fail. Sometimes you can get a little extra life out of the fan or at least minimize the noise by lubricating those bearings. Pull out the fan and at the center of the label you should feel a small depression. Use utility knife to carefully peel back the center of the label. Put in a couple of drops of light machine oil - a "3 in 1" type oil or sewing machine oil, whatever you might have handy. I wouldn't not suggest a WD-40 type product. Spin the fan by hand to work the oil down in to the bearings.
I've gotten months out of out a "lubricated" fan but I've also only gotten a few hours. Mostly it seems to depend on how long you wait between noticing a noise and doing something about it.
Those who are interested in Mikie's suggestion be sure to digest the second sentence. Historically I have never known it to be recommended to use a basic petroleum based lubricant on an external electric motor bearing. A polymer based lubricant is recommended.
The thought is that the housing, the bearing, and the industrial lubricating pad, if there is one, will instantaneously become "dust magnets" if petroleum based is used.
However, as a short term solution the action is of course viable.
First I would try cleaning everything out with canned air, getting the dust rhinos out of there can make a change. Using a q-tip to get into the smaller spots can work too.
Using compressed air (outdoors of course unless you are the one cleaning the home) is a very good idea. However it is very important to place a finger or thumb on the blades of the fan to prevent it from spinning wildly.
That cleans the dust bunnies but does nothing for the fan bearings which Mike K was referring to.
