Extremely noisy computer

I’ve tried everything. I’ve tried uninstalling things and reinstalling them. I have like, less than 25% of my hard drive used, and it’s only a 512 MB one. The only thing I haven’t tried to do to make my computer stop making so much noise is take it to the shop because I don’t have the money to spend on it.

What it does, is if I turn it off for a while and turn it back on, it’s fine. Well, not really It hums quietly. Then if I have a few programs running, it goes insane and starts howling really really loudly. Even the quiet humming is loud. and then if I shut down every one of my programs it gradually gets quiet again. So I can’t really run it without giving myself a migrane.

What do I do, and what is wrong with it? Does it need a new hard drive or what? I’m willing to save up some cash to get it fixed, but I want to know what’s wrong with it first.