Q: Whats a computer without power?
A: A boat anchor.
The first part of the computer I am going to talk about is the power supply.
What is it? The power supply is an electrical transformer that takes house current (120 volts AC) and turns it into 12, 5, and 3.3 volts DC, as well as other voltages depending on what hardware is hooked up.
Where is it inside the computer? When you take the cord from the wall and plug it into the computer (or any electrical appliance or gadget), you are plugging into the power supply.
Why does it make noise, why does it have a fan in there? Power supplies are noisy because they must have a cooling fan to keep the internal components from overheating. The faster electricity moves through wires and circuitry, the hotter those wires etc. get. If they get too hot, the components could melt or otherwise fail.