Ok technician has replicated the noise, got it happening now on both mac & pc & their awaiting advice from Allen & Heath. Hopefully they'll have some news for me next week.

I'm finding that as soon as I connect it to my mbp via usb I get a spike to -3 on the channel leds & then I'm left with a hiss like static noise. Phonos are silent, this only is on the soundcard channels, 2 & 3 are the worst, 1 a bit less & 4 is acceptably quiet. My faders are at -3, cha...

I know this is an old post but I'm experiencing the same issue with a Xone 4d & although I've sent it in to have it looked at, I was wondering how you resolved the problem.
I'll be sure to post what the technician finds out is the problem with mine for anyones future reference.