I changed nothing about my configuration and it just started happening. It’s a regular squak you can hear once every second. HA isn’t accessible anymore from the frontend. Automations probably don’t work either. I never had this problem in 2 years of using the same hardware.
Is it in a case with a fan? I had a squeaky fan on a 4b once that I had to unplug to keep it quiet. You could also unplug the Zigbee USB to see if that stops it. Otherwise it’s either the power supply or the Pi itself, or a cicada!
No fan, just a regular cooling grill thing. I tried unplugging the Zigbee USB stick but that didn’t help. I didn’t spot an insect inside the case. I’ll buy a new AC adapter and see what happens.