My computer runs really noisy, help?

My computer (Dell Dimension 4700) runs really loud, a whirring noise. It runs louder whenever something is loading or frozen. When its idle, it just kind of purrs, but if you click something it gets louder, than quiter when It fully loads. If you shut it off for a few hours, it runs quiet for about 20 mins, than starts to run noisy again. Any ideas?