Buzzing Noise

[Evil_Maniac From Mars]

Well, it was running silently mid-use, then suddenly started to buzz, loudly, and there was a small noise I couldn't identify, and then I flipped the power switch. It just started again, a louder buzzing/humming noise. Built up, died away. It's not the fans, I turned them off and it continued. It sounds almost like a jigsaw or skillsaw cutting through a thin piece of wood. When I power off the computer, it gradually dies down to a slow whine (like powering off a power saw), and then goes away. It doesn't happen with the computer on standby. I'm not sure if my fan controller has power over the PSU fan, so that might be it, but I don't know.