My apartment is vibrating. I suppose under certain conditions I wouldn't complain about it, but right now it's kind of freaking me out.

It started as a tingle in my feet, then I noticed that the water in the glass next to me was rippling like I was about to be chomped by a T-Rex. Various fixtures in the apartment began to rattle slightly with audible noise. I looked out the windows and saw no trucks or large vehicles. The vibrating dissapated after about a minute, but it happend again just now. I stepped outside. I couldn't feel or hear anything out there. I checked the basement, there don't appear to be any lose pipes or anything.

So this all leads to the $50 question: What the fuck are my upstairs neighbors doing?