Whistling Burglar Alarm?

Whilst unarmed, Galaxy 16 Model 605 alarm whistles for approx three hours, during which the keypad LCD screen is blank and green power light is constant, but the keypad unresponsive. After the whistling stops, the entry buzzers sound as though entering the property and the alarm can be deactivated in the usual way. There's nothing to match this description in the user manual. Does anyone have any suggestions/