Noisy Fan Fix
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.