Short bursts of Hi-pitched noise through ADAT

I’ve got an MOTU828x as my soundcard and I’ve hooked up a 896HD over adat for more channels.

Initially I had it set up with one adat cable and the 896HD as the clock source and this worked fine. However I thought it would probably be better if the 828x was the clock source and bought a second cable. The set up with 2 cables works fine however I am now getting stabs of hi-pitched sound occasionally - normally when the mac has just started up or when loaded cubase.

It sounds like the kind of sound you get when resetting the sample rate - but it can be loud and obviously I don’t want to damage my speakers.

Any ideas?