That depends really on your BIOS menu options. AFAIK on some you can set a max speed. Have you tried going through them?
In any case, I wouldn't recommend lowering the speed of your current fan. If you want to reduce noise levels get a fan that provides the same cooling but runs quieter, or consider something like water-cooling. The problem is that if you lower your fan speed, your components might overheat, something you definitely don't want.