If your noise is a high pitch whine then it could be electrical noise. The PWM on some motherboards tend to coil whine when the CPU is under load because of the PWM having to deliver extra wattage to the CPU. As much as I hate to recommend it, stick your finder in the cpu fan momentarily, not for to long that your system overheats, just long enough to see if the noise continues. If it does then you have coil whine on your motherboard & the only way to fix that is with a replacement. If the noise doesn't persist then you could possibly replace your fan.