|If desktop, open the side and without touching, look and listen to where the sound is coming from. Most likely the sound is coming from your case fan our your cpu fan and probably caused by a build up of dust or possibly a fan's bearing is going. If dust, shut down and pick up a can of compresses air from computer or office supply store, use the can to blow out of the machine (use a vacuum only to clean up outside of the machine). If needed, a case fan will be easily replaced, but the cpu fan may be separately replaceable, or may be integrated into the heat sink (post back for details on properly replacing the cpu heat sink).|
If a laptop, there is only one fan, one moving part, so just shut down, turn over, open any ports you find, and use a can of compressed air to blow dust out of all ports and air vents and externally, vacuum out the normal vents only and the mess from the blown dust. Close up and try it out again. With a laptop you probably will need to get service if the fan needs replacing (depending on your tech and repair experience level).
You have to be a little bit crazy to keep you from going insane.