Ive just gone to site, seen the issue first hand and fixed the issue. The issue was that in the AVR zone (see below) there was a "static" sound over the audio.

Im not sure "why" its fixed by doing this but this is what i did :

The audio system is built like this : Autonomics MSM2a (streaming sources) --> Audio Control Director M6800 (Power Amp, Matrix, Pre Out (for AVR zone)) --> AVR Yamaha RXA670

There are 7 distributed audio zones powered by the M6800, and one surround sound / distributed audio zone powered by the AVR which gets a pre out from the M6800.

The M6800 has 2x digi coax pro outs, i was using output A and putting that into the AVR. I swapped the wiring so that we now use output B into the AVR, using the same cable and everything else. So the only thing that changed was the output.

This seems to have fixed this issue, but i have no idea why. Any ideas ?