Where does the whirring sound of a computer in normal use come from? is it the hard drive or the cpu?

Usually the hard drive is so quiet you don't hear it. The cpu fan might start making noise if there's a lot of dust. It could also be the disk drive, power supply, case fan, and if you have one, the graphics card fan. Most times it's a fan running faster trying to cool when dust build up restricts air flow. You can pull the side cover to try to pin point the source. If you have a lot of dust, get some canned air to blow it out. Just make sure the system is unplugged and then press the power on to discharge the capacitors.