When there is knocking on a door in a dream it can be connected to an opportunity that is knocking (so to speak) in real life. It can also mean you are feeling an urge or a calling to try something in real life. Perhaps someone has been requesting some of you. Time to ask yourself if it is time to answer the call.
Often, whoever or whatever is on the other side of the door or is the source of the knocking will symbolize the opportunity or symbolize what you have been asked to do in real life. Be sure to ask yourself if the knocking annoyed you in the dream or if it made you curious or anxious. This will he