What is the causes of vibration in automobile engine?

What is the causes of vibration in automobile engine?
When the ignition is turned on, the engine starts running. But the shaft will not be connected to the wheel axle. Therefore, the vehicle will not move. But since the pistons inside the vehicle move continuously, the power delivered by the power stroke is dissipated as vibrational, sound and heat energy. The whole vehicle is subject to the vibrations as they don't have fixed, non-moving parts.
When the gear is brought to the right mode, the shaft connecting to the flywheel comes in contact with the wheel axles and thus the vehicle moves. When the vehicle moves, the power goes to the wheels and thus, no vibration is felt in the vehicle.