A loud computer fan might be caused by the accumulation of dust, damage, improper ventilation or a hot environment. Anything that keeps a computer's internal temperature high will contribute to fans working constantly to bring it back down, thereby leading to more fan noise.
Computers use several fans to maintain proper working temperatures. These include power supply fans, processor fans, graphics adapter fans and case fans.
Improper ventilation is one of the main reasons computer fans have to work extra hard and generate more noise. This is especially true of laptops, and can be resolved by placing laptops on hard and flat surfaces that are not made of cloth. Furthermore, users should clean all vents regularly with compressed air.
Users can attempt cleaning the inside of a desktop by opening the case and blowing out the dust with compressed air. If fans are still noisy, it may be an indication that the temperature inside the case is still too high. Users should have the computer serviced.