Popping sound on HDTV 37" ENVISION TV - only on HD Channels?

In my opinion the popping sound is audio dropout, either from poor reception or interference. I have experienced the very same thing when viewing thru a Set Top box. Pull up the menu and see what strenght the signal is for that station, you may very well notice that the reception strength varies, hence the popping sound. You may need to check all cables etc from aerial to to RF input, sometimes simply disconnecting leads & reconnecting helps.