I've been using an Apple TV 4k for 2 weeks, having given up on waiting for fireTV 4k fixes. Using Channels DVR on a powerful Linux server, and wifi network is fine (50Mbps). Unfortunately I'm finding that when a channel first plays it very often will have tiny glitches in the sound, almost like a static click. These will normally be randomly, e.g., a few over the first 5-10 seconds. Every time, if I then skip backwards and re-play the last few seconds of the channel, then everything is perfect. So that suggests that the data coming from the HDHomeRun is fine.
Also, if after changing to a new channel I press pause for a second, and then continue to play back, the glitches do not occur. So it seems to only be from playing in real-time.
I am in the UK, and this happens on SD channels very badly, but also on HD channels, but less obvious (smaller duration glitch).