Sources of Vibration
The main source of torsional vibrations in the entire drive train is the non-uniform torque output of the combustion engine. In addition, the transmission and drive shafts also act as sources of vibration. 
Engine
The tangential forces, periodically acting on the crankshaft journal consisting of gas and mass forces of the individual cylinders accelerate or decelerate the rotational motion of the crankshaft. These variations in the angular accelerations periodically create fluctuating relative angular movements (torsional vibrations) between the individual torsional masses. Further, the mass moment of inertia of the reciprocating engine is not constant. The linearly running piston and the swinging motion of the connecting rod gives rise to a periodic vibration of the mass moment of inertia. When excited by these mass forces, the actual forces that result depend on the amplitudes of vibration.
All those speeds at which the angular frequency of the forces of excitation generated by the engine match the natural angular frequency of the system are considered critical (resonance).
However, not all points of resonance are significant. The critical nature of vibrations can only be decided based on the deflections resulting from the respective vibrations. Mass and spring forces maintain the equilibrium in the resonance zones. The magnitude of the amplitude of vibration only depends on the effective force of excitation and the available damping.
Transmission
The sum-total of the vibration behavior of the drive train is also continuously changed by the masses, stiffnesses, variations in tooth rigidity, and damping of the engaged gear. The excitation is essentially generated by the constantly fluctuating spring constants of the teeth during tooth engagement and in defective teeth.