Click. Click. Nope, that's not a fan whirl you're hearing - that's the sound of your hard drive dying. If you're unfamiliar with the sound of the hard drive death trance, we suggest sharpening up your detection skills with this quick and easy guide.
"If your hard drive makes noises like these and you are still able to access your files - backup immediately."
You can access their database of sound recorded hard drive clicks (given you know where to find the manufacturer). If you have no idea what make your hard drive is, you can grab CPU-Z (Windows only) to help identify the make of your drive.
After working in IT for a few years, I can tell pretty easily the difference between a dying fan or a hard drive failure, although most people tend to mix them up all the time. The trick is to listen for persistent low frequency bumps followed by clicks. If a fan fails, it usually doesn't make any noise at all and causes your computer to freeze constantly due to overheating. That's when you can instantly tell it's a fan issue rather than a hard drive one.
However, poking around your PC or Mac with a screwdriver isn't usually the best idea if you haven't done it before in the past. We usually recommend bringing it to the Genius bar or a computer repair shop and have them take a look at it to confirm your diagnostics before going online to purchase a brand new hard drive.