Here is my setup:
-Intel Mac Pro Quad 2.66 with OSX 10.4.11
-MBox2 (firmware version 1.43) running ProTools 7.1.1cs2LE
-MOTU 2408mk3
-Cubase 4
-Mackie 1202
-Samson amplifier and Alesis Monitor One speakers
-Audio Technica studio headphones

-Mbox2 connected to Mac w/supplied USB cable
-Left & right monitor outs on MBox connected to 1202 with balanced 1/4" cables
-Left & right main outs on 2408 are connected to 1202 with balanced 1/4" cables
-1202 connected to amp and amp to speakers
-Headphones connected to 1202

When I open up ProTools this annoying high-pitched whine is generated. I hear this in my speakers and in my headphones. However, if I connect my headphones directly to the MBox headphone out this noise is not present. Also, if I set the MBox to be the default out for my Mac, any time I try to play some audio, the noise appears. This also happens in Cubase if I have the Digidesign driver selected.
I am not getting the noise generated from my 2408 however. (I did get some noise before, but replacing the cables fixed it.)

I searched online for a solution and couldn't find one specifically for the Intel Mac. There was something about setting the NAP on the Mac with CHUD tools, but that was for the G5.

I swapped out the cables running from the mbox and that didn't help. I tried several USB cables and still no dice. I also disconnected everything except Mac, monitor and 1202, but still the same issue. I also tried running everything from the same outlet like one post mentioned and still nothing.
If it was a ground issue, wouldn't it happen with the 2408 as well?
I also removed ProTools and reinstalled, starting with my LE 7.0 disk and then my 7.1.1 update (req'd to work on Intel Mac). Still got the noise, so installed the cs2 update and still same issue.

The last thing I read was about getting the Monster Ultimate Performance USB cable. I can't get one locally, so I'll have to order online. But I'd rather have that be the last solution. Don't want to spend $25-$30 for something that won't fix my issue and I can't use later.

Now the noise isn't a problem for loud music, but I do a lot of subtle orchestral stuff and it's really annoying and interferes with my editing.

Would upgrading to PT 7.4LE will solve it? I have had the unit for years, so it's out of warranty, so if it's a defect, I guess I'll have to live with it.

Any solutions would be appreciated.