HELP...Computer Gurus

It is a gateway computer if that helps any. I did open up the case earlier and had a look around. The fan is running fine and also when you shut the cpu off, you can hear that "winding down" sound(not the fan, but something else) that you hear on most computers. Sounded like that noise came from a decent sized "box" inside the computer at the top in the rear of the cpu....sorry for the laymens terms, but I'm no computer person.