There's the relay click when changing inputs, but that doesn't sound like what you are describing. If I had to guess what you're hearing, it sounds like your 990 is un-muting the digital circuit just as it locks on to the digital input's format and you're getting a little pop as it locks. That signal lock process would happen any time the signal format changed (switching between channels that are playing different formats, or when the format changes for commercials, or probably when new levels load in a game). For what it's worth, I've not heard that on my system.