One of the most annoying things is fan noise in a computer. Computers have many fans. Most are fairly easy to change. You have to check out a few things before you can change the fan.
Fan size - This s usually given in mm eg120mm o 12cm
Voltage: The fans in a PC are usualy 12V dc but always check.
Number of pins in the fan pin header. You will need a fan with 3 pins if the system monitors and regulates the fan speed.
Also when fitting the fan make a note of the direction of the flow of air, this is usually marked on most fans by an arrow in the casing.