The first is that the fan may be dusty, and the second is that the fan is out of oil. Open the case and see if there is dust on the CPU fan. If you can use a small brush to brush it, use a hair dryer or leather bag. Just blow it clean. If there is no oil, tear off the label in the middle of the fan. There is usually a rubber pad under the label. Use tweezers or something pointed to pick it up and put a little butter inside.