When doing the first harvesting quests (or I believe any time when lumberjacking) there is a shake to the world when each time your axe hits the tree.
This shake continues (more like a vibration) for some time after you harvest. It seems to randomly stop on its own, or if you /sit or do something which changes "modes". This twitching of the world is annoying as heck and happens with a frequency of perhaps 2 or 3 times per second.