The spectre of a phantom knocking on the door could be an enthralling and confusing experience. It is often difficult to understand what is happening and why. It's also a phenomenon that is not well understood however there are a variety of explanations as to what may cause it.
Another chance is to believe that the sound may be an indication of paranormal activity. It could mean that a supernatural entity (or spirit) is trying to contact you. Or even trying to warn you of something. It is not uncommon for people to experience hearing knocks while they are at home, or when they're experiencing a moment of heightened emotional state. It could be due to an interaction between the physical and spiritual world.
Another possibility is that the knocking is an outcome of a process known as "pareidolia". Pareidolia occurs when the mind interprets randomly occurring patterns as identifiable shapes or forms. This is why we hear knocking at the door and not having one outside. The sound could come from many sources, including the sound of the wind or the outside traffic, or even from the house itself getting settled. The brain interprets the sound as knocking, and then imagines there is someone at the door.
A different possibility could be that it could be simply a figment of one's imagination. It could be a result of anxiety, stress or boredom. The mind of a person can be tricked by their mind and it's easy to think of things that are not in reality. This can be especially relevant when one is in a quiet space and their mind is free to wander.
watch Knock at the Cabin may be the result of something more mundane. It's likely that it is coming from the neighbor's home or from a nearby street. It is also possible that the noise is from an animal such as a bird or a mouse, which is trying to get in.
No matter what the cause of the mysterious knocking, it can be terrifying. It is helpful to remember that it's probably not a sign of anything supernatural, and will likely go away on its own. If the knocking continues then it could be beneficial to speak with someone who can help for help, like a therapist or a spiritual advisor. They might be able to give insight into what could be causing the knocking, or offer suggestions on how to cope with it.