The term 'vibrate violently' can be summarized as juddering, which is a 6-letter word. Juddering is a type of vibration that occurs rapidly and with great force, often causing objects to shake or shudder. It is usually caused by an imbalance in the rotating parts of a machine, such as an engine or motor. Juddering can also be caused by a sudden change in the speed of a vehicle, such as when a car suddenly accelerates or decelerates.