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.