The two biggest culprits for excess noise in computers are fans and the hard disk. Fans are used to move the heat produced by the processor, motherboard, and graphics card out of the computer. If the fans are loose, too small, or not powerful enough, they can create noise.
Computers can also make noise if any components have come loose and are vibrating against the frame of the computer. If the computer is in the wrong area, any noise from inside the computer can become amplified.
As computers age, they acquire dust, pet hair, and lint, which can block fans from running efficiently. Parts can also come loose, or just wear out, increasing vibrations and noise.