A quiet doorbell-like electronic tone
My 27" iMac has started sounding a very quiet tone at random intervals, typically every 15-20 seconds. It's none of the alert sounds, is not affected by the volume setting and even does it when sound is muted. I've closed every app, re-started in safe mode etc. OS is up to date. Nothing makes any difference. I have not added anything new to the computer but this has now persisted for 2 days. I have seen a similar question from 2019 but that didn't have a resolution as far as I can see. Any ideas?
iMac (2017 – 2020)