Do you have anything to lift the ground? With computers, usually this stuff is a grounding issue, even with high pitched noise. Usually I associate grounding issues with 60Hz hum and so forth, but it can be whine from like the computer fans or something and the USB powered audio interface amplifies it... you may or may not be able to hear some faint noise when you shake your mouse (before booting protools).
The fix may be to lift the ground to the amplifier of the speakers (a lot of people use Hum X).
You could also use a cheater plug to test it temporary, but if that fixes it you'll be best to use Hum X or something similar to have your ground.