Did you take the fan out and blew off the dust inside the rotor?
Mine used to make a noise so loud that I thought I forgot the pc on, but I took the fan out and cleaned it with izopropil alcohol. After I applied some silicon grease (not for fans/electronics, but for 6 months works like a charm) - not too much! just to have a very fine film on the moving surface.
If you do this and it still makes that noise I don't know what else you can do apart from buying a new fan.
Good luck!
