THUMP in the night

I woke up at 2:50 AM last night feeling like the main character from Poe's The Telltale Heart. I had been slowly dragged from sleep by an unwavering, incessant beat that had a strong/weak rhythm just like a heartbeat. THUMP-da, THUMP-da, THUMP-da, THUMP-da. I couldn't tell what on earth it might be. It sounded like someone playing a bass drum outside our house, or the low steady bass of a car stereo. It never faded, the tempo remained constant. Cody was still sleeping, so for a brief moment I wondered if I was going insane like the man in Poe's story. I decided to wake Cody because the sound was bothering me so badly. He investigated, but never discovered what it could be. He did find that it could not be heard from downstairs though, only upstairs. I finally went to bed downstairs and gave up on knowing where this sound was coming from. Cody told me that it lasted until around 5 AM this morning. Not sure why he didn't join me downstairs!