If your front door, or perhaps any other door has suddenly started making a chiming noise through your alarm system, it's because you have activated the 'chime' feature.
This feature is toggled on and off by pressing the 'C' key when the alarm is in it's normal unset state. When you press the 'C' key you will see a small lower case 'c' character appear or disappear on the keypad LCD display.
It's nothing to worry about. Note: Only zones/detectors programmed by the engineer to be chime enabled will trigger if you activate the 'chime' facility.