If you have a computer fan that sounds like the bearing has been replaced with gravel, there may actually be a simple fix. All you need is a screw driver, a small pick, and some lubricant. In this instructable, I repair my GPU fan. However, the process should be nearly identical for any fan in your PC.
Step 1: Remove Fan
I repaired my GPU fan. First, find the screws holding the fan in place, and remove them so that you can access both sides of the fan freely. Now is a good time to clean up all the dust that's been collecting around the fan. Some canned air will work nicely.
Step 2: Remove Sticker and Split Ring
First, remove the sticker on the fan. That will expose the shaft and the split ring that holds the fan in place. Use the pick to gently pry the split ring off the shaft and remove the fan. Set the split ring aside for reassembly later. If possible, leave the o-ring in place. Otherwise, put it on the fan shaft.
Step 3: Lubricate
I used some Hoppe's #9 gun lubricant on my fan. It worked great. Add a few drops of lubricant to the inside edge of the fan. Rotate it around until it looks wet. Try not to use more lubricant than necessary. It helps to put the fan back in the bearing and spin it around a bit to get a nice even coating.
Step 4: Reassemble
Be sure the o-ring is still in place as you put the fan back into the bearing and pop the split ring back on. Add another few drops of lubricant to the fan shaft around the split ring and spin the fan around a bit. It should be nice and smooth now. Reattach the fan and turn it on. If all went well, it should be nice and quiet again!