I could have sworn that I saw my OPN print version of the instruction manual has this, but I’m not seeing it on the electronic version I just checked, and I don’t have the printed version manual with me right now.
But I’ve experienced what you heard myself and that’s why I looked up my user manual and found it there, albeit on my printed version only, I guess.
What it is is a warning that your microphone needs servicing. The reason is unknown, but I noticed that it happened to me when the weather was really humid and I started sweating excessively when playing tennis outdoors. Mind you, it only happened on one of my 2 OPN 1s, but it’s exactly like what you said (4 sets of 8 sounding chimes), and that’s also exactly what I found in the manual. The manual said to return it to your hearing provider to send in for servicing.
Like what you said, it continues to work just fine after the chimes, but the chimes keep coming back randomly although often enough. And in my case, usually when I go outdoors in humid weather. I took it back to my audi and she tried cleaning the mic ports for me but it didn’t make any difference. She finally had to send it in and the problem didn’t go away until after they fixed it for me. Presumably just replaced the mics on that OPN. They might have replaced the mics on the other OPN as well, I’m not sure, but I sent in both of them to have the problem checked out because I have an old pair of hearing aids to wear for backup.
It was a good month or two after the chimes started before I could send the OPN in for servicing, and all that time, the chimes kept coming back from time to time.