Recent progress in noise and vibration analysis technology had made great contributions to both noise level reduction and sound quality improvement in the interior noise of passenger cars.However, in spite of remarkable reductions in interior noise level, the sound quality in diesel passenger cars is still judged to be worse because of its different sound characteristics compared with gasoline versions.By using subjective testing, it was found that the main cause of poor sound quality in our test vehicle was the high contribution of relatively low frequency half-order multiple components, principally 2.5 and 3.5 order of engine rotation. The undesirable vibration transfer characteristics of the chassis was found to be one cause, but the half order components of powertrain vibration were also shown to be at a high level, and were the source of the excitation. Our investigation, which used new analysis methods, has identified an excitation mechanism for the half order powertrain vibration which has not been previously reported.In order to evaluate the vibration shape of each order at engine running condition, we made use of Operating Deflection Shape analysis (O.D.S), using proprietary and in-house written software. It was found that, below the resonant region of powertrain vibration, the half order multiple components were composed of rigid body motion plus torsional elastic deformation, whereas at integral orders the shapes had predominantly rigid body motion.At high frequencies, natural mode shapes of the powertrain were excited in all orders and show close agreement with the natural modes obtained by a static modal analysis.The excitation force raised by inertia force and torque fluctuation was measured during a single cylinder operating condition. The vibration responses and the relation between these results and O.D.S. with all cylinders operating has also been investigated to find the reason why the half order components have torsional deflection shapes.