Hard to tell for sure, but an initial guess would be that the fan is starting to go. If you aren't using your optical drives, then the only other possibility is the hard drive is starting to have issues.
First thing to do: Make sure you have a full and complete backup, immediately. Anything you cannot afford to lose, make sure it is backed up now. Hopefully, it won't be needed, but better safe than sorry.
After that, it would be worth cleaning your computer to see if that helps. Given the way cooling on a computer works, the fan draws in air, and thus all kinds of dust and junk, constantly. Think about fans that are in regular use. I know we have one in our bedroom - the cage outside the fan and the fan blades themselves get really dirty. It's mostly just dust, but it accumulates and can really mess up a fan over time.
With a computer, most people never think about that fan on the inside getting dirty. Even if you dust the outside of the computer regularly, it likely has accumulated a lot of gunk.
The best way is to open up the case and use canned air to blow it out - concentrate on the fan itself, but blow out any dust you see inside the case. Be careful not to touch any of the circuit boards - most times, nothing will happen, but just a little static electricity can really fry the electronics.
If you don't want to open it up, you can just use some canned air & blow it through the vents on the computer - but it's likely you won't get it cleaned out very well that way.
If the fan has just gotten noisy because dirt and dust have accumulated to the point of interfering with fan operation, a quick cleaning should get rid of the noise (and help keep the gunk from killing the fan!)
If it still makes noise after that, it's probably worth replacing the fan. Fans tend to be cheap, and if it goes out, the computer could overheat and really fry the computer before you realize it. (Note: if the fan noises suddenly stop, especially if the fan has ALWAYS been noisy, don't just think, "Oh, good, it fixed itself" and ignore it. Turn off the computer immediately - it may be that the fan went out.)
If you've done all that, and still have a problem, you may need to replace the hard drive. Again, they aren't that expensive. Keep good backups (i.e., daily incremental backups) until you replace it.
If any/all of this is beyond you, do get someone to help. If you have a geek friend/family member who you know is decent with this kind of stuff, it's all pretty basic. Enlist the help or offer to pay them! If you don't know anyone and need to have it done, look for small, independent computer repair shops that have good reviews on-line. That's going to be the cheapest place to get it done, and the most likely to do it honest and well.
Avoid the big box repair places like Best Buy. They will charge an arm and a leg for the help. These are pretty basic things to do, so they probably wouldn't screw it up, but you'll pay a premium, and may end up having them tell you it's not salvageable & try to sell you a new one.
