1. Some computers may be "24V DC Cooling Fan" of the configuration is not high, the boot when the sound is relatively large, the latter will slowly become smaller, which is a relatively normal phenomenon.
2. If a long time fan or loud fan rotation axis may be rusty, can be less wipe some oil on the shaft, lubricate.
3. There may be because the "24V DC Cooling Fan" blew something up, you can open the fuselage to see.
4. Dust the radiator fan behind the plate and should always clean up
5. The fan blades "eccentric" is the main reason for the big noise, you need to re-install the fan securely.
6. If these do not make noise a big problem to solve, which shows "24V DC Cooling Fan" is working. May be a problem with the hard drive, you can find a professional look.