plug and play with a penetrating problem

the item has been delivered as ordered and arrived in the promised time frame - all good.
After trouble connecting to the TV (with optical audio output) and a converter (for
Conversion to a digital signal for digital audio in via coax to surround system) and the appropriate configuration
the TV's to use external speakers, everything went well and as expected (Dolby Digital 5.1 works).
However, there is a problem: when you switch to another TV channel is a loud hissing / popping sound output to the surround sound system, the cause of which I previously could not narrow. For a tip I would be grateful for this purpose; if
the cause is related to the Toslink cable, is incomprehensible for me, I can also not rule out.
Not - the mute button on the remote control of the surround sound system and after switching the TV channel a De-Mute serves as a workaround
nice, but better than loud hissing / popping sounds.