Hi Nicky,
Welcome to the forum.

I've been using the Central for several years now, and have not heard the symptom you are describing. However, I don't use the optical cable nearly as much as the network connection with an attached Rio Receiver.

It sounds like you've covered all the bases by swapping things out, that's what I would try as well.

So how often does the noise/sound occur, and what exactly does it sound like?

Thanks, Randy