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.

There are only two main noise sources in your computer and they are the cooling fans for the power supply and CPU, and your hard drive.

CPU fans are usually almost silent so the hard drive and power supply sound like the probable culprits, power supply fans often get noisier as they age.

The hard drive can't really be quietened and the only option is to replace it with a more modern drive that runs quieter.

How old is the PC? You may want to try defragmenting your hard drive, that can sometimes help.

Also, check how much free memory you have available to see if it is almost full.