I hit the treadmill at the hotel the other day since I was really in a hurry in the morning. It was a modern LifeFitness model, the ones that often have the TV attached (this one didn't), and I was surprised by how shaky the treadmill was. If I ran in the middle, it shook audibly, and visibly, making it hard to hear the TV and unnerving me a bit.
I started running at 2.0 incline, but backed that down. I even went to 0 and it was better, but still shaky. I noticed that the left side was much worse, and if I slid to the right a bit, essentially running right of center with both legs, it shook much less.
I did that, and worried because occasionally my shoes would squeak on the side of the treadmill as they hit the deck. Eventually I was close to center, just enduring the shaking. It was an annoying and hard run, making me wish I'd gotten up 30 minutes earlier and gone outside.