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Here's the kicker - when the machine is running on its battery, it's fine. It's only an issue when running off of AC.
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To me, it sure sounds like a power supply problem. When there is no load on the power supply, the power supply does not generate the interferance. Put a load on it, and it does.
I've never looked into one, but the supply is likely to be some sort of switching power supply, and they are inherently noisy.
Why yours would suddenly become unable to filter out the noise at the same time as your reformat - I have no idea. I can't see how they are related.
Wild coincedence maybe?
Check out: http://computer.howstuffworks.com/question352.htm
But if I recall correctly, those beads mentioned do their best work as the frequency goes higher and higher. I suspect that if your noise is related to the power supply, it would be low frequency.... unless it is a harmonic.