A study is made of internal excitation mechanisms which are at the origin of medium and high frequency noise in Diesel engines. The theoretical and above all experimental methods used are presented. Finally, the means likely to efficiently contribute towards the reduction of acoustic radiation are broached.Among these mechanisms, special interest is paid to combustion, piston slap and vertical excitation through piston, connecting rod and crankshaft. For that purpose, simultaneous pressure and vibration measurements are made along the reciprocating gear and on the walls of the crankcase. Thus, after adequate processing applied to the signals collected, the result is a complete description of all these mechanisms.This description is then compared with results coming from implemented models which, at the present stage of their development, enable a few significant parameters to be scanned and their influence and scope to be deduced.