Hearing a knock on your door can represent a feeling, desire, or fear that:
Someone wants to communicate with you
An opportunity is available to you
Someone is trying to get access to you in some way, such as to interact with you, understand you, or get information or something else from you
Someone wants you to "open up," be honest, or connect with them in a real way
Someone or something is threatening to "invade your space," such as compromise your integrity or take advantage of you
You're expecting or
Thinking about an opportunity