Because it has power. Unplug it and it will be quiet.

NO poop unplug it!!!!!!!! I already know that, I had another computer and it is not noisy at all.

You really didn't give enough info on this.  Is it a new computer, old computer..??  If it is old then there are fans that could be maiking the noise.   Try stopping one of the fans and see if that stops the noise.   Also check to see if you have a CD in the Rom Drive.  I heard some computers that sound like a 747 taking off if you have a cd in the drive.