When I turn on the controller, this "beep" sound appears.
What can it be?

Sounds like a stick error. Have you connected to the app to try
to identify the problem?

Beeping like that can be stopped by simply refreshing both craft and RC FW, you can do both in dji assistant.

Dirty Bird Posted at 7-20 00:39
Move the sticks.  One is being detected off center.  If it stops, perform a joystick calibration.

How do you perform a joystick calibration?  I could find no reference to that in the manual.  My controller does at times power up and beep like that.  Usually if I power cycle the controller it will come up the next time OK.

Dirty Bird Posted at 7-20 10:34
Attach device to controller, leave aircraft powered off, in Fly enter camera view, go to Settings > Control > RC Calibration, follow prompts.

Thanks much Dirty Bird!

Thank you very much, I will calibrate the joystick

Hello there ESchimidt. I am sorry for the trouble this has caused and thank you for reaching out. Please try to calibrate your DJI Mavic Mini's remote controller for us to be able to isolate the said issue. I will be posting an official DJI tutorial video on how to calibrate the DJI remote controller. Please keep us posted for further assistance. Thank you.

Thank you very much, problem solved.
The error was the lack of calibration.