An interesting problem has arisen with some dual-processor G5 computers, where PSU-generated noise has ended up affecting connected Firewire audio devices, as well as being physically audible from the computer itself. Having the machine upgraded to a Revision E PSU usually fixes the problem, or at least improves it, but there are stories of the problem persisting even then. When I had mine upgraded to a new PSU, the Firewire noise was cured, but the physical noise from the PSU is still audible and tends to change as you use the mouse. One possible fix is to use a patch called CHUD (part of Apple's free development tools) to disable processor napping, but Apple seem to discourage this even though those who've used it don't seem to be reporting problems. You need to open the control panel and disable napping each time the machine is switched on, and if you're cautious you should probably only invoke it when recording audio or mixing.
When working on a laptop equipped with a Firewire audio device or DSP unit (such as TC Powercore), it may also be better to record audio to the internal drive rather than to an external Firewire device, as this places less demand on the Firewire buss bandwidth. You can always back up to the external Firewire device after recording. If in doubt, do a track count test with both the internal and external drives, and see which gives you the most tracks of recording or playback without glitching.
