The sound you're describing certainly sounds like the fan. This sits on top of the CPU (aka the "chip") and, as you might imagine, keeps it cool.
Computers can indeed get very dusty inside. You can purchase special "computer hoovers" for want of a better term to get rid of this, but most people just blow it away with a canister of compressed air. However, you will indeed have to open it up to do this.
If you are at all uneasy about this, I would suggest taking it to a local computer repair shop...