some of the G5's have a problem that sounds similar to yours. they put it down to something about the frequency of psu and cpu not getting along with each other or something... maybe not same as your problem but there's an article on the apple site that you cud find by searching for noises +g5.

Use balanced signall path if your not already, that'll probably help.

trouble shoot it out! try other interfaces, headphones, different locations.

ground loops?

sorry i can't be of more help

goodlcuk