There is this old legend that says if you hear three knocks that seemingly have no cause, someone you know will die. Silly, right?
Many believed that door knockers are spirit, who knocks your door in the middle of the night. It is believed that, whenever they come and knock your door, somebody will die on the next day.
There are many legends of death knockers all around the world. Here's the Philippine version:
The Kumakatok (door knockers) are a group of three robed figures believed by many in the Philippines to knock on doors in the middle of the night, bringing bad omens. They allegedly look like humans but wear hoods which obscure their faces to some extent. One resembles a young female while the other two look like old people.
A visit from the Kumakatok is usually an omen of death, as either the eldest or an ill member of the house visited will subsequently die. The visits are supposedly more frequent after a disease outbreak. Residences of Luzon and Visayas at one time painted white crosses on their doors to ward off the Kumakatok. This trend was said to cause the trio to switch from residences to government buildings, hospitals, and even churches.
Reported sightings of the Kumakatok have decreased significantly since World War II. One explanation is that many buildings were destroyed at that time, leaving the Kumatakok few doors to knock upon.
[Reference: Paraiso, Salvador; Jose Juan Paraiso (2003). The Balete Book: A collection of demons, monsters and dwarfs from the Philippine lower mythology. Philippines: Giraffe Books. ISBN 971-8832-79-3.]