So the recent Mac Pro audio update 1.0 (applied on 10.6.2) has reduced my CPU temperature with audio playback (Presonus Firebox Firewire audio interface) and that is great. Also haven't had any machine crashes (grey screens) recently which is also great.
The problem is that when the Firebox is plugged into the firewire port of the Mac Pro there is a constant high pitched whistle of slightly oscillating frequency. If I unplug the Firebox the whistle goes away in a few seconds.
After prolonged use in a quiet room (the kind of room you need for audio recording / mixing work!) the noise is enough to drive me crazy.
You can read the thread here.