With the computer off you can unscrew the PSU and case fans and set them outside the case. It's much easier to tell if they are making the noise if they are outside the case. Turn the computer back on and hold them up to your ear, if the part is making the noise you'll be able to tell. Be careful, it really hurts if you stick your finger in a fan. If it's not one of those parts, you can do that with the graphics card (disabled) and HDD as well. If those pass as well, it's probably the motherboard.