great info peepz,
lets sum this up (digital or analoge summing?

)
It seems the noise:
- is platform independent (OSX/XP)
- problems vary (or disappear!) when using different computers with the same unit.
- only comes up when the DAE, ASIO or Coreaudio driver is engaged.
- apparently gets bounced to disk (although I did not noticed that myself, but I work in Cubase SX3, so maybe thats a different story when bouncing, non-realtime etc.)
- Changes when using an outboard clock (extreme jitternoise maybe??)
Let's complete that list with some more info, so we have something Digi can work with to fix this, cause up 'till now they do not recognise this problem.
What I would like to see tested, but can't, is to:
-Try it on a powered usb-hub, I know it ain't recommended, but if it has something to do with supplying current, that test might clear it up.
Don't worry, it won't blow up your Mbox2, if it does, u'll get mine instead (but I must warn u, mine has a noise problem

)
- try an outboard clock on the s/pdif and clock to that, Vocalvoodoo already pointed out a change in noise, lets see other units try that.
We'll break this thing, don't worry. If else we are providing valuable info to Digi to find a fix or at least a cause to this problem.
pzz