Electronic doorbell system, seems to be broken.
To alert you when visitors are outside your home wishing to enter, a doorbell will do a much better job than a thunderous knock. Originally, doorbells were operated by pulling a cord that sounded a bell. Modern doorbells are electric or wireless and are triggered by a simple push of a button. Their sounds can range from a simple bell to an electronic apartment buzz, or even a short musical tune.
Found in most houses, wealthy estates, and secured buildings.