When I moved in here, I had a troupe of clog-dancing elephants above me. Nobody since has been quite as loud, although there's been varying degrees of noise.

The most annoying one was the guy who, every Saturday around 11AM or so, would do something in his living room that made the floor joists go SQUEAKYSQUEAKYSQUEAKYSQUEAKYSQUEAKYSQUEAKYSQUEAKYSQUEAKY
SQUEAKYSQUEAKYSQUEAKYSQUEAKYSQUEAKYSQUEAKYSQUEAKYSQUEAKY
SQUEAKYSQUEAKYSQUEAKYSQUEAKYSQUEAKYSQUEAKYSQUEAKYSQUEAKY
loud enough that it actually scared my cat. One day I got a broom and pounded on the ceiling, and the SQUEAKYSQUEAKY slowed down and stopped, and it never happened again. He moved out a few weeks later, I think.

I have decided to believe it was exercise equipment.