Originally posted by CritMasterFlex:

You can get a pretty violent screen shake effect sometimes when standing on uneven ground. It's the same shake that happens when you're sliding down a steep incline, only you're not moving. I too thought the ground was shaking when it first happened to me.

It happened to me once when I was standing in a small "V" depression on top of a terrain wall I built. It was a brief wtf moment but I recognized the sound effect from the hillside surfing.