Noise-vibration-harshness (NVH) engineering specifications in vehicle design are being defined in order to establish that the vehicle is commercially acceptable. The NVH quality of vehicle powertrains depends on its individual offenders. In order to reliably identify these main offenders, two novel hybrid simulation models have been developed. This paper demonstrates the benefits of this engineering approach in order to effectively enhance the transmission sound quality and consequently the vehicle powertrain NVH.Transmission noises can be separated into two groups. The first group deals with noises from unloaded transmission components such as loose gears or synchroniser rings bouncing inside their functional backlashes and clearances. The second group represents transmission noises resulting from gears engaged in the torque flow, i.e. teeth engagement noise.In this paper we present results obtained by our basic noise research on the vibration of gears. For the non-linear rattling simulations a novel powertrain torsional vibration propagation model has been developed. All relevant components of the powertrain topology from the engine to the wheels are implied. Non-linearities of the system such as the clutch hysteresis and the gear backlashes are also implemented. Regarding the teeth engagement gear noise we defined a dynamic model of two masses considering 12 degrees of freedom. It describes the engagement of two helical gears. Both the transmission error and the variable stiffness during the engagement have been taken into account.The reliability of predictions with these two models has been verified by means of vehicle tests.