Have you cleaned it with compressed air?
I have an Acer Aspire M3100-u3201A desktop. Today when I turned it on, the fan was very noisy. I tried rebooting it and then unplugging it for several hours. When that didnt work, I tried disconnecting and reconnecting the fan in case something was loose. Its still making noise and its driving me crazy. Until today, it has been extremely quiet. I am not a computer expert so I dont know what else to try. Anybody know how to fix this?