DC motor operation with a varying voltage
The motor experiences torque ripple as it commutates and the coils switch. 

Usually the PWM is fast enough that the motor inductance smooths out the current, and torque is proportional to current. If the PWM frequency is low you can get 'vibrations' that cause audible noise from the motor. For example, some older electric streetcars made whining noises when accelerating.



The really old ones used carbon pile controls and were relatively quiet (but wasteful of energy).