One major noise source of diesel engines are timing gear impacts. The structure vibration excitation mechanisms of the timing gears of different 3-cylinder diesel engines are investigated. To this end, the engine timing gears are equipped with acceleration sensors for simultaneous measurements of the rotary gear motion. The peripheral acceleration component shows impacts between the different gear pairs which are due to the alternating camshaft torque and alternating fuel injection pump torque. The dependence of these excitation mechanisms on e.g. engine speed, load, start of pump delivery and valve clearence will be described.Additional findings on the impact generation are obtained by analysing many consecutive cycles of the peripheral gear acceleration.By comparing the peripheral acceleration measurements taken on the rotating gears with the structure vibration measurements on the gear bearings, the impact transfer into the engine block structure is roughly quantified. On the basis of these data, acoustical behavior can be estimated.The influence of gear train modifications, including the toothing, on structure vibration excitation mechanisms will be discussed in order to point out effective measures aiming at an acoustical optimization.