It sounds like the receiver is popping everytime it locks on to the dolby digital signal when you change stations. Try only hooking up the analog cables or switching the input mode to analog and see if it still happens.
Need Help with Popping Noise from Speaker
I'm hoping someone can help. My HD sateillite receiver is connected to my Home theater preamp. When I watch a HD channel and switch channels I get a pop noise emitted from the speakers. It seems to happen only when changing channels from a HD station to another station. Any ideas what could be causing this to happen?