Popping sound --question

I have an NAD 752 A/V receiver, but only have it set up for stereo at the moment, with two front speakers. It is connected to a Hughes Directv/Tivo receiver via digital optical cable and s-video. I noticed that when I go to a TV channel that is broadcasting in Dolby, I hear a slight "pop" as the receiver automatically switches into 2 channel Dolby mode (at least I am assuming that is what is causing the pop, since it only happens when I switch to a channel broadcasting in Dolby). Does anyone know why this would happen? I have checked the conections, and they all seem fine. Will it go away once the system is set up to fully take advantage of Dolby Digital?

Thanks.