[voiceblue]

Happens to me as well. Now I just ignore a knock on the door (or rather, most any sound) if it happens just once as I'm waking up. It has been a hallucination every time.

Bonus: If someone is at my door at some ungodly hour, they might leave me alone.

It's pretty remarkable how real it sounds though, complete with being in the distance - just enough to make you think it came from somewhere else in the house (usually the front door) - it definitely takes a deliberate act of will to totally ignore it.

Since another commenter has mentioned a knock on a window (distinctly different sound), I wonder whether our brains just contextualize this to whatever is plausible.