"Thump! Thump! Thump!" from the downstairs neighbors.
Since the family who live down there consist of a 20-something young mom/nurse, her elderly mother, a chihuahua named Poco, and a boy around 6 years old, the ideas as too what's making this sound are limiting.
But here's my idea of the source of noise:
* Little boy has to help pay rent on the apartment by entering a clog dancing competition. If he doesn't win, they will have to sell him to Calvin Klein as a model.
* Older mama is running a meth lab and the thumping is a series of small explosions from the endless glass beakers and containers. She always has a smile on her face, and that's just not natural.
* Poco is really a weird dog like in The Hulk, one minute he's a trembling chihuahua who barks endlessly at falling leaves and my parked car, another minute he's 50 times as big and bouncing off the walls, hence producing the thump thump thump sound.
Anyone else have an idea?