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Diesel engines have become increasingly popular as a power unit for passenger cars over the last decade, due mainly to their superior fuel economy. The first generation of high speed direct injection engines suffered from a lack of refinement by comparison with contemporary spark ignition engines. Analysis of subjective assessments of typical diesel engine noise identified impulsive sounds and the excitation of low frequency resonances as the major reasons for adverse subjective reaction. In addition torsional resonances in the drive train can be excited by fuelling variations as a result of interactions between hydraulic waves in the fuel injection drillings and pipes (time dependent) and the pump filling and injecting events (speed dependent). Drivetrain resonances can be excited also by rough clutch engagement, road imperfections and sudden increases in fuelling in response to driver demand.