A TV or receiver will faithfully produce the signal you send to it. Since the uverse box is where the channel change occurs, it's the part that is sending the scratching noise.
Good to see you describe this so well. Makes it a lot easier.
i have a samsung 65" 8000 series tv, uverse tv, and a bose lifestyle 28 system. when the tv is set to external speakers, and a dolby digital 5.1 spdif setting, i hear a very annoying loud scratching noise when changing channels. this noise goes away when i change the spdif setting to pcm. DD5.1 gives better sound quality but has that annoying noise. has any one had a similar problem and how have you solved it.