Your computer probably produces a moderate amount of noise whenever it's turned on. The CPU itself operates silently, but the fans and disk drives can produce enough noise to create a serious distraction when you're trying to concentrate. In many computers, the case itself can amplify the amount of noise created by vibrations from the fans and drives.
Therefore, the inside of your computer's case might contain some insulating material to absorb the sounds produced by spinning fans and vibrating disk drives. This could include foam or padding along the inside surfaces of the case, or special rubber or plastic supports and grommets to isolate vibrating fans and disk drives from resonant parts of the case. If you add your own insulation to the inside of your computer case, be sure you don't interfere with air flow.
Unfortunately, most case makers don't pay enough attention to sound reduction, so your computer probably makes more noise than it should. Chapter 27 offers some suggestions that can help silence a noisy computer, in spite of a bad original design.