With the reduction of engine noise, the gearbox noise becomes more audible. Therefore, it is necessary to establish multidisciplinary simulation models based on Multi Body Dynamics (MBD) for transmissions in the same manner as they are applied for vibration and acoustic analysis of the power unit. In the past mathematical simulation of engine and transmission was carried out independently. More and more the consideration of all interdependencies of the entire power train plays an important role already in the design phase to achieve the optimum 'system behaviour'. Based on the individual target of the simulation (e.g. component analysis, system performance modeling) it is necessary to employ simulation tools of different complexity. This paper deals with the increasingly necessary system simulation of entire power trains consisting of engine and transmission. It is focusing on the NVH influencing parameters at the transmission based on flexible multi body systems co-simulation to outline the way how to achieve 'virtual testing'. A simulation model is shown which represents the dynamics of the gearbox for simulating gear rattle noise as well as gear whine noise. Variable meshing tooth stiffness and nonlinear damping in the backlash are considered for all gear pairs, including the final drive and differential. Using this method it is possible to introduce additional parameters related to the gear noise as design parameters in the overall design process. The integrated solution enables to analyze and minimize the noise emitted from automotive transmissions in the early design stage reducing time and cost compared to experimental tests. The simulation methodology was developed in joint collaboration between Elasis and AVL.