allthatglitters,
Since it is the Fan on the Heatsink, you have two options:
[list=1][*]Remove the 4 screws that hold the Fan on the Heatsink, take the fan apart to clean it. You can use something like WD-40 to lubricate it and but it back together.[*]Buy a new Heatsink/Fan to replace your current one.[/list=1]
Personally I would get a new Heatsink/Fan. There are some new ones that are very quiet, and others that are very high performance. (Should you want to overclock your computer.)
Take Care,
Richard