Most HDDs have a feature known as SMART which is basically advance warning of impending disk failure. Strangely, most brand name systems ship with this feature switched off, so the first thing you might want to do is to enable this in your BIOS if it's not already.

If the disk is indeed what's making the noise, then I would look to replace it before it flat out dies on you. You've already backed up important files, which is good, so you'd be free to reinstall Windows on the new drive once you get it. There is no reliable way to back up Windows or Windows programs anymore, and that is by design. So you should be gathering together all your installation/driver CDs as well.

But before you get too excited, it would be a good idea to try and pin down exactly what is making the noise. It could just be that a case fan's bearings are starting to go out, which is often a much easier and cheaper fix. You can test this by opening the case, while running, and finding the fan. Use your finger to stop it spinning (it won't hurt a bit) for a second and see if the noise stops. If it's not the fan, try and pin down as best you can where the noise is coming from. No point replacing a part that isn't defective.