A CPU is the "central processing unit" which is a usually a small flat square sandwiched between its motherboard mounting and the heatsink/fan assembly. I've heard folks use CPU to mean the entire box to which mouse, keyboard, monitor, etc., is attached to. Is that whole box what you are calling the CPU? If so, I could guess the noise comes from your hard drive but they generally are not that loud. When you run a program, the hard drive is being accessed and will make some noise. I'm presuming you've already opened the case and stuck an ear to the interior to listen for the noise. I could offer that I've sometimes used a cardboard tube such as from an empty roll of paper towels or such to help me isolate a noise. Stick it up to your ear and listen around in the case but don't let anyone see you doing this.