Fan Basics

Smaller computer fans must spin faster than large ones to get the same result. Your case fans may spin very fast by default, and a central processing unit or video card fan can be particularly loud.

Managing Case Fan Noise

You can plug your case fans into a "fan bus." This can be a small dial added to the cable, like a dimmer switch, or you can plug your fans into a box that sits in a 5.25-inch bay like a DVD drive. Each dial will control the speed of one fan.

CPU fan

Fans included with a CPU purchased at retail are generally 60 mm (measured along one side), and these can be quite loud. Such fans can be replaced with a variety of choices, some ranging over 100mm, which are much quieter.

The Video Card

Fans included on gaming-oriented video cards will get very loud when processing 3D information, since they essentially use a turbine to push air directly out of the case. These fans can be replaced as well, but the quieter solutions to not externally eject air.

Hard Drive Noise

A spinning hard drive sounds much like a fan and can be distractingly noisy even when idle. There is not much you can do to force the drive to be quiet. If it is a secondary idle drive, Linux users can spin it down with hdparm. Apple OSX and Windows users can configure power management settings accordingly.

Source:

Overview of case fan tech

Lowering hard drive noise

Quiet computer guide (with video)