There are only a few things that can make noise inside your computer. One common thing is the power supply fan. You can test this by taking a pen or other object and while the computer is making the noise stick the pen in the fan in the back to stop it momentarily to see if it affects the sound. If it does then you may want to consider getting a new power supply before the fan goes out and overheats the computer.
It may also be the processor fan. You can take the cover off the computer and start it to make sure the fan is running on the processor. You can do the same thing by momentarily stopping the fan to see if the sound stops (Don't do it more than a moment). You don't want to have your processor fan go bad because it can quickly ruin the processor. Even if its spinning it could be spinning too slowly and overheat.
Finally your hard drive could be making the noise. All you can really do is try and listen closely when the cover is off the case and see if it's coming from there. If it is then it may or may not be a problem. Sometimes you will have a noisy hard drive and it will be fine and sometimes it will be on its way out. If you want to play it safe you can backup your data and replace the hard drive but keep in mind you will have to reinstall Windows and all of your programs.