I don't have a solution yet, but here's more data from some experimentation I've been doing. The bottom line is that in my case at least the problem MAY be caused by a poor fit. If I get a resolution I'll return to this thread, but for now I'm offering this up in the hope that it may help someone else's investigation of the problem. Here goes....
Rob bought new AirPod Pros in order to use the new hearing aid feature. He fitted the silicone tips using the fit test feature, took the hearing test, and turned on the hearing aid feature. As soon as the hearing aid feature was enabled, random chirping began in both right and left units. I call it random because it occurs at irregular intervals varying between 2 seconds and 20 seconds, there is no correlation between right and left units, and there does not seem to be any correlation with sound in the environment. The chirping disappears when the hearing aid feature is turned off, and returns when it is turned on.
THE CHIRPING IS CAUSED BY THE HEARING AID FEATURE.
Rob borrowed Louise’s AirPod Pros, paired it with his phone, inserted it in his ears, applied the hearing test profile from his test, and turned on the hearing aid feature. The chirping appeared immediately, and disappeared when the hearing aid feature was turned off.
THE CHIRPING IS NOT CAUSED BY THE SPECIFIC AIRPOD PRO UNIT.
When Louise put either AirPod Pro set into her ears and turned on the hearing aid feature using the configuration from Rob’s hearing aid test, she did not hear any chirping. (She did clearly hear the changed frequency response so we know the hearing aid feature was working.)
When Rob put either AirPod Pros into his ears and turned on the hearing aid feature, Louise could hear the chirping when she put her own head very close to Rob’s - that is, it was loud enough to be heard from outside.
We took pictures of the AirPod Pros inserted into both Rob’s and Louise’s ears. It is very apparent that the AirPod Pros do not fit into Rob’s ear canal as far as they do in Louise’s. For instance, the black oval on the top of the unit is partially exposed in Rob’s case, but is wholly within the ear in Louise’s case.
If Rob presses hard on the units to force them deeper into the ear canal, it SEEMS like the frequency of chirping is reduced, though this is hard to quantify, and in any case as soon as the pressure is released the units shift and the chirping frequency resumes.
THE CHIRPING MAY BE ASSOCIATED WITH THE FIT.
