I have since noticed it goes off every 20 minutes exactly, three times an hour, although not every hour of the day.
It’s extremely intrusive but I can turn down the volume.
I have checked every notification setting on every app, gone through every possible system setting but cannot find from where it is originating. I have rebooted, installed the latest system update, all to no avail.
I work in IT so consider myself fairly knowledgeable but this is defeating me. I can find no other reference to this on Internet.
Any ideas, please??