Since it's happened twice now, I suppose I have to report it as a clear bug.
The other day I was using my newly purchased "Toy Story 2" 4K UHD Dolby Atmos BluRay to see how the built-in factory BBC 9.1.6 Atmos room sounds, coupled with my own Stax HPEQ. The audio "settings" on the disc were for Dolby Atmos, but my HDTV at this location is an older Panny 65VT50, which is of course not 4K. I was using my Oppo UDP-203, with its video-only HDMI1 going to my Yamaha AVR and then out to the TV, and with its audio-only HDMI2 going to HDMI2 of the A16. I know that only HDMI1 on the A16 is fully HDMI 2.0 and HDCP 2.2 compliant, but I'm not passing through video. So I don't think this is significant for my problem report. So I expected to still hear the full Dolby Atmos sound from the disc, even though I was sure this 1998-era film couldn't possibly have had more than a 5.1 or 7.1 original underlying sound track even if this new disc offers Dolby Atmos. And in fact, the "info" button on the Oppo says it's reading "Dolby TrueHD 7.1" from the source, even if its sending "bitstream" out over HDMI2 to the A16. I don't know why it doesn't say Dolby Atmos, but this is my first disc with Dolby Atmos audio so maybe this is just the way the Oppo reports it.
Anyway, I navigated to Chapter 22 ("When She Loved Me" song) and after being sure I had the volume where I wanted it as well as confirming (using TEST mode) that I could SOLO and hear all 16 speakers, I switched the A16 to "Audio Meters" mode. I wanted to see if the disc was actually only sending 7.1 audio, or were there more channels in this Dolby Atmos implementation since the BBC listening room on the A16 was capable of 9.1.6. Turns out it really was only 7.1, with no signal showing on any other channel.
So, I continued to listen for a while (still in Audio Meters mode). All of a sudden the sound turned into a GIANT BUZZ!! I had a panic attack, and thought it was coming from the Oppo. So I pushed STOP, but the buzz persisted. I tried to use the volume-down rocker on the remote, but of course the volume rocker is inactive in Audio Meters mode (it's only active in speaker map display mode). Next thing I did was push the POWER button to shut the A16 off, and the buzzing finally ended. I was certain the A16 was the source of the buzz, rather than my external DAC (fed from optical out of the A16) or the Stax amp.
After waiting a bit, I started things up again, but did not have a repeat event. So I mentally observed it, and was prepared for it to show up unexpectedly in the future.
Yesterday I watched about 8 consecutive hours of Netflix (DD5.1 audio) streaming via my Oppo BDP-103 (set to output audio as LPCM, not BITSTREAM as the 203 is) connected from its audio-only HDMI2 output to HDMI1 input on the A16. For all 8 hours of operating in a 5.1 PCM room on the A16 (my AIX 5.1 PRIR coupled with Stax HPEQ) sound was perfect, fed LPCM from the 103. No buzz.
Tonight I decided to learn how to compose a listening room from arbitrary speakers selected from multiple PRIR's. So I used as a practice example creating a new 9.1.6 Atmos room, starting from the existing 9.1.6 BBC PRIR and then overlaying the 7.1 ear-level speakers from my AIX 7.1 PRIR. Thus I was combining my primary AIX 7.1 PRIR with six overhead and LW/RW speakers from the BBC 9.1.6 PRIR. I figured that would be a pretty good way to turn my 7.1 AIX PRIR into a 9.1.6 Dolby Atmos environment. Seemed to all go ok, and using TEST/SOLO mode I was able to prove that all 16 speakers were working properly.
Then once again, I put the "Toy Story 2" disc into the 203 and once again played Chapter 22. Once again I got my volume right, and then went to "Audio Meters" mode to watch the levels on the 7.1 channels. Well, it didn't take but three minutes for the GIANT BUZZ to reappear!!! Again, I had to power the A16 off to save my ears.
So this buzz has now occurred TWICE, and only when using BITSTREAM input into a 9.1.6 Atmos room, fed from a Dolby TrueHD 7.1 audio, and while operating in Audio Meters mode (although this may not be as relevant as the other factors).
I have no solution. Just my observation that the identical GIANT BUZZ (which eliminates any real audio) has now occurred twice, very early on after starting, when playing the identical BITSTREAM Dolby Atmos audio source through the Dolby Atmos decoder in the A16, and two somewhat different 9.1.6 room setups (one pure factory PRIR, and the other from my combining 7.1 AIX + 9.1.6 BBC). I have not experienced this buzz using LPCM input heard through a 5.1 PCM room even after 8 hours of continuous listening.
Just saying. That's my report.