Krissy

Hello! 3 times now I was sitting in my car on break, not driving, and within a minute or two a REALLY loud bang happens in my driver side door. Not sure if its outside or inside. It literally sounds like someone punching my door. It only happens once (that i noticed) each time I start my car. Im not sure if it has anything to do with it being a cold winter or my heat being on high. But it has happened 3 times now and I didnt see anything that would have hit my car. Any clue on what this could be?