The reason for this sound; the freezer door is left open. If the freezer door is left open, the door open alarm will begin within a few minutes. You will hear a warning sound that will make you aware that the door is not fully shut. As soon as you close the door, the alarm will turn off.

This indicates that the Fast Freeze function is activated. The Fast Freeze function on Defy fridge freezers allows for rapid freezing and quicker ice-making. It’s normal for your refrigerator to run for longer when Fast Freeze is on. Fast Freeze should not be left on for longer than 5 hours. If you want to cancel it, simply press the Fast Freeze button until the light goes out.

The sound you're hearing is the door open alarm. If the refrigerator door or doors are left open, the door open alarm will be activated in a few minutes. As soon as you close the door, the alarm will turn off.