Many modern systems have a supplemental case fan, and some have more than one. The more loaded the system, the more supplemental cooling you’ll be required to use.

While selecting fans consider following:

Case fans are available in many sizes, from 60mm to 120mm or even bigger. All other things being same, a bigger fan moves the same amount of air with less noise than a smaller fan as the bigger fan spin slowly. Obviously, the fan mounting positions in most cases are of fixed size, so the options of size are limited. If you have choices go for the biggest fan that fits in.

Case fans generate different levels of noises, even for the similar size and speed. Many factors effect like blade design, type of bearings, grill type, etc. Overall, ball bearing fans are noisier but durable than fans that use needle or sleeve bearings.

The level of noise can be reduced by running the fan at a lower speed, as long as it is providing proper cooling. The easiest way to reduce fan speed is to install an inline resistor to reduce the supply voltage to 7V. These can be created by using a resistor from Radio Shack or other electronics supply store.

Some fans have a control panel, which mounts in an available additional drive bay and lets you to control fan speed constantly from 0 to maximum by setting a knob. Some fans are created to be controlled by the power supply or a motherboard fan connector. These fans differ their speed automatically in response to the ambient temperature, running at high speed when the system is loaded heavily and generating lots of heat, and at low speed when the system is free.

The mounting method you use makes a difference. Most case fans are protected directly to the chassis with metal screws. This transfers vibration directly to the chassis panels, producing noise. A better way is to use soft plastic snap-in connectors in place of screws. These connectors cut off vibration to the fan itself. Better still is to use the soft plastic snap-in connectors in conjunction with foam around it that insulates the fan frame from the chassis completely.

The above mentioned 3 steps are the main problems to consider in order to reduce noise from your PC. After minimize noise from those major sources, take the following steps to reduce noise from minor sources.