More Buzzer issues

I have APM 2.8 .(as far as firmware goes same as 2.6 “they” say).
Buzzer seems to work but not according to manual.
I get a long beep as it is arming…not a problem…good warning as the props start to spin.
One short beep if it won’t arm for some reason.
One short beep when disarming.
Low battery alarm seems to be working as advertised.
BUT, every once in a while, on the bench, it will start giving out 4 beeps…3 short and 1 long…and it will keep doing this until I unplug it. It doesn’t seem to matter if the tx is bound or not.

Only power is battery…using telemetry to connect to MP.

Any idea what the 4 beeps are for ??