To start, allow me to say that the VAVA Support Team (RavPower) has been great at helping me with this sound issue. But not that it has surfaced again with a NEW unit, I thought it best to share my experience in hopes that someone in the community knows how to resolve the issue.
When watching the unit at some random interval, it will start making a high-pitched screech sound. This was the same issue that I had before, and VAVA kindly exchanged the unit under Warranty for a replacement. Yesterday, I hooked up the replacement unit, downloaded the latest update, and started to watch TV via the Apple TV connected to it. As soon as the Netflix program started, the sound was present. So I shut down both units and thought wow - a brand new unit - maybe the problem is the HDMI cable. So I connected a different (new) cable, and four minutes later the sound was back. I then thought that perhaps it was the Apple TV. I brought in a second Apple TV that had been working just fine in another room. I used yet another HDMI cable and yes, the sound came back.
I did discover however, that when the HDMI is impacted via the settings, changing to HDMI 2.0, enabling ARC, and so on, the sound stops; for awhile. That “awhile” duration could be 4 seconds or the remainder of the week, based on previous experience.
UPDATE: I just performed a “Factory Reset” and that fixed the problem. I hope it lasts!
UPDATE (13 minutes later) The sound is back - so I am posting my videos of these events here so hopefully someone can diagnose the issue.
UPDATE: I think the root cause may have been diagnosed! I will continue testing over the weekend and post my findings later.