|You failed to give specific information on your system. Therefore I can only give you general things to look into.|
1- Download Speedfan and check your internal temperatures while you are running. A high core temp might mean dust build up on cooling fins and/or case vents, an improperly installed heat sink, or poor case air flow. Just to name a few.
2- Look in your BIOS settings to see if your fan settings are set to automatic.
3- Is it actually too fast, or is it just getting too noisy? Post your temps and your fan RPM (just after starting, 1/2 hr. later, and after 2 Hrs. of continuous use). If your fan's bearings are going, it may be getting noisy and may be warning you to replace it before it fails.
Your readings over time will help use help you as well as detailed specs (or model number if major brand) and other details like how new, how long this has be coming on, and any other problems (even if they do not appear to be related).
You have to be a little bit crazy to keep you from going insane.
If all else fails, read instructions.