Hi,
Go to Setups>Hardware Setup and make sure that your clock is set to internal. Does the same thing happen if you load and play the demo session? Is the noise only during playback or does it also occur when Pro Tools is not playing back? Are you using a dedicated audio drive and does this happen if you use your system drive instead? Have you tried moving the Mbox to another USB port on your G5? You may also want to try updating the firmware in the Mbox to make sure that it is healthy. You can find the updater here:
http://www.digidesign.com/download/mbox/
Let us know what you find,
Thanks,
Digidesign Tech Support