|The description of the sound is not very detailed but it could be:|
Hard drive is going (back up your files first, then investigate further).
One of your fans' bearings are going or something is rubbing on the blades.
There is not much in your computer that has moving parts other than these. Open your case (as above mentioned) and listen carefully. While you cannot 'see' what is actually making the noise, your two ears will make you turn your head so that your eyes are pointing nearly directly towards the noisy component so you kind of 'see' what the problem is. While you should always power off and unplug your system before actually touching internal components, IF you are VERY careful, GROUND yourself FIRST to the metal of the case, ONLY, CAREFULLY touch the hard drive case (not near the connectors) to feel for the vibrations and the same for the OUTSIDE of the fan HOUSINGS (being VERY careful to AVOID the blades). This will help confirm your 'spotting' the problem by it's sound alone. Of course if you see a fan blade actually vibrating, that will not be needed. If you have a bad fan, replace it. If the problem is your hard drive then make a comprehensive backup of your files, and if possible make an image of the drive while it is still working to an external drive for later when you can transfer it to a new drive (easier than reinstalling from scratch) or clone the drive directly to a new drive. Both are not difficult using the software available for download from the hard drive manufacturer of your drive or the new one you purchase.
You have to be a little bit crazy to keep you from going insane.