Knocking in the dark

After coming to a foreign town, you decided to find a room in a small inn.
At night, your sleep is disturbed by persistent sounds behind the door.
When you came to the hall to find out what was going on, you saw that there was no one behind the door.
But you can’t go back, because the door has slammed shut.
Now you must go downstairs for a spare key, but to do so is not so easy.