In order to minimize power train length, it is required for transmission components to layout compact. The requirement results in thinner and larger transfer gears, which are disadvantageous for gear noise. On the other hand, improvement of gear accuracy and/or vehicle interior noise increases sensitivity to manufacturing variation. It tends to appear noisiness by transmission unit variation.To prevent such gear noise problem, we made detailed investigation by both several tests and simulation, i.e. noise measurement, shaker test, running gear vibration measurement and FE Model analysis.This paper describes the experimental analysis of gear noise generation mechanism of transmission with thin and large diameter gear and its prediction method. It was found that gear web out-of-plane vibration modes are closely related to vibration transfer to the mission case. Planarly non-symmetric modes have dominant effect for case dynamic excitation by gear engagement. Gear meshing analysis with FE method gave good correlation between calculated vibration and measured emitted noise quantitatively. By using these techniques, we developed a new noise reducing technology which can be applied to limited space gear train.