Hearing knocking with no one there
Hearing knocking when no one is there is unsettling because a knock is the one house sound made to mean a person. Pipes, wood and animals make it often, and the mind fills in the visitor.
What it may mean
Of all the noises a house makes, a knock is the hardest to ignore. Creaks and hums blend into the background, but two or three sharp raps sound like a hand, and a hand is someone wanting you. When you open the door and the porch is empty, the mind keeps searching for the visitor it was promised. That reflex says something about what you were already expecting. People who are waiting for news, missing someone or bracing for trouble hear knocks differently from people who are at ease. Old beliefs in England, Germany and Belgium read unexplained knocks as warnings or news of a death. The sound itself is most often the house talking. What your thoughts jumped to when the knock came may be the more useful thing to notice.
Depending on how you saw it
Exactly three knocks
Three is the number people remember, partly because the mind groups sounds into patterns and three feels deliberate. Pipes and cooling ducts tick in short bursts, and the ear readily hears three of them as a set. What pattern have you been reading into a run of chance events?
Taps in the walls after dark
When the heating switches off or the night turns cold, metal pipes and ducts shrink and slip against their supports with sharp taps. Wood framing does the same as it cools. Which part of your day makes the most noise as it settles, and what helps it go quiet?
A knock at the front door, nobody on the step
A delivery driver already back in the van, a branch against the storm door, children playing a prank: the empty step tends to have a dull explanation. Checking the doorbell camera or the street often solves it. When did a mystery dissolve as soon as you went and looked?
A thud when someone turns off a faucet
A bang in the walls just as a faucet or washing machine shuts off is often water hammer, the jolt of moving water stopped suddenly. A plumber can fix it with an arrestor. Which jolt in your routine comes from stopping too abruptly rather than easing to a halt?
Why it happens in nature
Most unexplained knocks come from the house itself: pipes and ducts expanding and contracting as temperatures change, water hammer when a valve closes quickly, or wooden framing shifting as it cools at night. Mice, squirrels and woodpeckers can also rap on walls and siding. Still nights make these sounds stand out.
What folk beliefs say
West Country, England
In England's West Country, a woman whose son lived in America took three knocks at her door as a sign that he had died.
Source: Cora Linn Daniels; C. M. Stevans (eds.), Encyclopaedia of Superstitions, Folklore, and the Occult Sciences of the World, vol. 1 (1903), vol. 1, p. 195. View the page
Germany
German lore told of the Klopferle, a knocking spirit believed to live in every house, heard but never seen, that knocked before a death in the family or some other ominous event.
Source: Cora Linn Daniels; C. M. Stevans (eds.), Encyclopaedia of Superstitions, Folklore, and the Occult Sciences of the World, vol. 3 (1903), vol. 3, p. 1397. View the page
Belgium
In Belgium, strange knockings on the wall, in the cupboard or in the wardrobes after dark were taken to show that one of the family would die that night, and the number of knocks told the hour.
Source: Cora Linn Daniels; C. M. Stevans (eds.), Encyclopaedia of Superstitions, Folklore, and the Occult Sciences of the World, vol. 3 (1903), vol. 3, p. 1398. View the page
To reflect on
When you heard the knock, whose face came to mind first?