Gears play an important role in many mechanisms and aggregates, providing the transmission of rotation and torque. The development of technology places increasingly high demands on transmitted loads, reduced vibration and noise, increased strength, durability, wear resistance and smooth operation of gears. Therefore, the problems of vibration and noise, which are caused by many factors, are still key in many cases. The rigidity of the gear engagement, which periodically changes with the frequency of reconnection in the engagement, causes the occurrence of inertial forces acting with this frequency on the contact and bending dynamic component of the loading of the teeth and bearing supports. This effect is especially strong in resonant zones, the position of which is closely related to the rigidity of the supporting elements and the reduced mass of the gear train. The paper considers some aspects of this influence and shows the simplest case of the functioning of a gear train. However, it quite clearly demonstrates the peculiarities of the effect on the gear pairs loading and the vibro-acoustic characteristics of the rotational speeds transmission and loading moments. It is advisable to consider these features both when choosing the most rational technical solutions for the development of measures to reduce noise and vibration of gear-based transmissions, and when using vibration acoustic monitoring to assess the quality of manufacture and assembly of gears.