Who is at the door?

Who is at the door, I wonder, as I have been rattled from my deep sleep.  I am frightened to death.  For the past few weeks, I have been awaken from a deep sleep of someone knocking at the door.  My heart is pounding in my ears and my eyes are as wide as they can possibly open in the pitch black bedroom. 

My mind tells me where I am and that I am alright.  I am able, after a few minutes, to pull myself out the bed and go from window to door looking for signs of someone possibly knocking at the door or window.  I also see nothing.  So I return to bed and calm myself down and try to think about what I was dreaming about when the knocking awoke me.  I can't recall a dream.  I finally drift back off to sleep. 

The last time I heard the knocking noise, it was quite frightening.  It was not a knock but a pounding on the door.  I jumped up this time and almost lost my breath.  I was too scared to move for it was so real and fear had paralyzed me.  I was sure someone was at the door this time.  I waited for what seemed forever before I got up this time.  I went again looking from door to window in the house.  Nothing. 

This has happened four times to my knowledge.  I have not had the courage to tell my husband.  I have not heard it in a few days.  However, my family situation has changed somewhat.  When I was hearing the knocking noise, it was just my husband and I in the house.  Now, my twin daughters have moved back in with us. 

During the times the knockings awoke me from my sleep, my inner self kept telling me not to open a door or window.  I have been very cautious not to open the house to any outside forces.  I am certain it was something, but what?  I have been doing a lot of praying and have had a lot of stress these past few weeks.  I have a lifetime of life experiences to fill a museum.  Remind me to tell you some tall tales sometimes.  However for the time being, who is at the door?