V+ : Speaker noise

[SILVER SURFER]

0ne for all you audio experts! I like to listen to audio played through my receiver to external speakers rather than use the tv's own speakers. I have a optical lead from tv to receiver for Freeview audio. However, when I switch from watching Freeview to Cable, pressing the AV button on the tv remote, there is a loud click in the speakers. Removing the optical lead solves the problem but leaves me without Freeview audio. For Cable I have a HDMI lead from v+box to receiver and another from receiver back to tv. If anyone knows what is causing this annoying click I would be grateful to hear from them as I would love to solve the mystery. Many thanks.