An unstable foundation
This next story is one of assuming one thing – but finding out another one. There was no train nearby, nor was there a race track or any other reason the apartment should have shaken, though. It shook a lot, however.
She later found out that it was due to an unsafe foundation. This is something you do not want to hear from the place you are staying in. “Oh yeah, it does not have a stable enough foundation, which could cause it to crumble.” This all sounds fairly scary, but at least the renter got to the bottom of things.