Hi Mark,
I have the same with my setup, in my case I think it is because I have used a home made link cable and the screening is not good enough between the data and sound cables. I used multicore cable that only had one overall screen and I think other members may confirm that is not correct as there should be screening between the digital and audio pairs. The sound you hear will be the interference from the burst of digital data that is being sent via the commumications cables. I routed my cable through walls and a floor (so a big pain to change it) so I have just put up with it as it only happens when changing something. Next time I will use better cable :)
In your case you could run another cable along side just for the data wires or replace the whole lot the correct cable, good luck.
Steve