High pitch squeal related to Firewire audio device after 10.6.2 audio patch

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.

I read a post on another forum with other people having the same problem. Can anyone comment here on seeing the same thing? Is there any known work around or fix for this?

When the CPU is under heavy load (e.g. H.264 compression etc.) the noise subsides only to return when there is normal load (i.e. half a dozen apps running).

Help!
Thanks