http://www.arstechnica.com...g5-noise.html
It's been about 6 months since the Power Mac G5 was introduced, and Apple still can't seem to produce a decent power supply for its dual processor models. There are several noise problems related to the dual G5's power supply
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user input actually influences the noise. Showing and hiding the Dock makes a nice, brief chirp. Doing network copies makes a sustained squeal. Booting makes a squeaky warble. When idle, there is a small chirp every second or so.
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For shame, Apple. For shame.