Upon a Midnight Dreary

"...There came a tapping, as of someone gently rapping"
Well, it's not at midnight, but the sound is the same.
    It started sometime last fall and always at night, waking me from sleep. I hear the same sound: KNOCK, knock, knock.   Always 3 raps with the first the loudest. Once we had a crazy blue jay who used to pound on the side of the eave at the back of the house, but never at night. I know it's not the cat. She sleeps through the night.
   Now I know that my premise might be wong. I hear 3 raps that wake me up or so I think. But could it be that there were previous raps while I was still asleep. I don't think so, though, because the sequence is always the same, one rap followed by  pair of slightly quieter raps. It sounds like the sounds are coming from near the front door.
    I always get up to check. The first time I thought it might be Nellie, as she has knocked on my door occasionally, though never at night. Of course I look out the window to see if a vehicle of any kind is in the driveway or on the road in front of the house.  Maybe someone had a problem and didn't have a cellphone with them. Maybe the house is on fire, smoke somewhere that someone reported. Or, more worrisome, maybe some intruder is rapping to see if  I'm awake before he pushes the door in. That has happened twice before. No one was home, but I imagine the burglars knocked before breaking in.
   The rappings are infrequent. I would say about every other month or so. Last week, I was awakened to  the familiar series of raps. It was 2:00 a.m. KNOCK, and then knock, knock. I was so tired I didn't feel like getting up to check anything. I told myself that it was the same as the other times. Looking out the front window would accomplish nothing. So I lay in bed, but after a while, I had to get up to look. Nothing in sight in front of the house, cat sleeping on the top of the couch. Nothing to see here. So I went back to bed.
    But out of the usual mode, the same thing happened only 2 nights later. This time at 3:30 a.m.  I know it's not my imagination. I got up to check as always, nothing there as usual. I try to think; maybe that rapping is connected to the water softener booting up. Or the furnace getting ready to turn on. I don't know, have no choice but to go back to bed.
So I ponder, weak and weary.