In diesel engines, combustion is found to be a significant noise excitation. However, the aim of the present study is to introduce a concept by which all the engine noise sources can be classified. A special test rig has been installed for this reason which enables simultaneous acoustic measurements, in addition to those related to thermodynamic measurements. The procedure for accurate determination of direct and indirect combustion noise is explained based on a 3D multi-zone steady-state interior-exterior acoustic problem. The predicted direct combustion noise is considered to be due to combustion noise point source within the engine cylinder enclosure, and according to boundary element method (BEM). Results indicate that the proposed combined experimental-prediction approach is a reasonable approximate technique for the separation of mechanical, direct combustion and indirect combustion noise from the total engine noise. However, the influence of some thermodynamic parameters such as ignition timing, valve timing, compression ratio, air/fuel ratio ..etc can be investigated in future, and consequently better control for all the engine noise component can be made.