A standard design feature of a lot of ESC’s is to beep when it does not see a ppm signal.
This meant to warn you that the ESC cannot see the Rx or the Rx is switched off.
Even in the large T-Motor equipped Octo’s we get beeping of all 8 motors until the safety switch is pressed.
The motors on my two Tricopters and one quad don’t, but then again they all have the same make ESC, so I suspect that’s why I have not had the issue before.
I was starting to think it was something to do with having all the arming options turned off so all I have to do is press the safety button and its all ready to fly.