Burglar Alarm external alarm sounding

The battery in the bell box needs replacing

Look for external tamper switch on the bell first

The system requires servicing/ investigation to check for all possible issues. The external bell has an internal battery that may need replacing, it is also possible that the tampers have corroded inside the bell. Occasionally water may have entered the system causing shorts on the board. It is also possible that the battery inside the main panel is on its way out, a simple test with a battery tester unit will determine the health of the panels back up battery on most systems. Reason to recommend a service/ investigation is that many things could be the cause but to be certain needs investigating, alarm engineers may inspect and test the system to advise fully what is required, some provide this service free of charge during normal working hours but check with them first as some may have relatively high fees for the call out and inspection time.

This issue sounds like a case tamper on your external bell box. It may also be a battery fault - dependant on the age of the bell box.