Here is a well written post about the problem (from the now-defunct osx forum) http://discussions.info.apple.com/WebX?128@9.Bb3navPMhAu.5@.599a487d http://discussions.info.apple.com/WebX?14@140.RHXia7X9pU3.0@.599d2ccf Rob Yales email to logic-users: When you turn off the Nap setting in CHUD tools, you disable processor cycling. Because Apple put a cheap transformer coil in the power supply, the processor cycling induces RF which propagates throughout the G5. Check this out from the MacFixit article I mentioned: Reader Markus Altendorff has also posted a few excellent demo sound files, and provided a few informative comments: I can confirm the PSU noise problem - my machine beeps/chirps under any of the following conditions: a) Graphics card use, esp. with OpenGL b) Copying files over a network c) Opening a file in Quicktime player After installing the CHUD tools (Apple, Developer software), disabling "CPU Napping" (some sort of power-down mode during idle clock cycles) makes the chirping go away COMPLETELY and instantly. My guess (and backed by a friend of mine, former electronics pro and now IT...) is that (cheap) filter coils in the PSU start to vibrate, similar to loudspeaker coils, when the CPUs switch between high and low power use, which can happen very often at 2 GHz clock cycle. Since disabling CPU napping results in the CPUs consuming a fairly constant current, the vibrations stop (the magnetic fields in the coils remain constant). The bad thing about this is that the chirping also shows at the USB and Firewire ports, causing trouble when you try to use this computer in a recording studio - the thread at Apple Discussions contains quite a few complaints about how the G5 is barely usable for audio production because of this. Several readers report that Apple replaced the power supply in their G5 as a result of noise issues. In some cases this solved the problem, but not in every case.