I notice an electrical/buzzing sound when my laptop is connected to the mains. The noise isn't coming from the power supply but from the laptop itself. This noise coincides with CPU activity which leads me to think it may be 'CPU whine' but if that's the case I don't understand why there is no noise when running from battery. My model has SSD storage so not expecting any noise to be coming from the HDD. Any ideas as to what might be causing this? Thanks.