In the paper the possibility of the direct application of forcing computed function based on the elementary model of the diesel engine regarding issues of the rattle vibrations in modelling of gears was introduced. In this way the simulated indicator diagrams were created. After that, build diagrams were used to calculate tangential forces and torques operating in the drive system. Considering only the torsional vibrations, these calculated values of torques can be introduced as the forcing that are acting on gears inside of the gearbox. The presented in the article different models enable us to diagnose and analyse the neutral gear rattle. The forcing of the diesel engine introduced at the low range of rotation speeds is one of the most important parameters which may cause, within the clearance, the creation of the phenomenon of a lack of contact of the regular meshing of teeth, and, as a consequence vibrations and the difficult acoustic effects connected with rattle.