Fading sound and/orvoice, then popping

I have a Nakamichi AV-10 receiver. I have noticed that I will put on a CD or watch a TV program and the sound out of all speakers becomes muffled -- like it's fading away, or just the voice disappears. I have checked the settings to see if the sound is set on normal (not DTS, surround etc.) Then I will hear a pop that also occurs in all speakers, the normal sound resumes, then it quickly fades. What is also weird is that the pop can occur with the volume off out of ALL speakers. This is not a tube amp. Any ideas? It's driving me crazy.