An experimental study was conducted to investigate piston-impact generated noise in diesel engines. A coherence model was used to represent the noise generating mechanisms of the engine. The model was applied to an in-line turbo-charged diesel engine. Frequency response functions were measured between the cylinder liner vibration and the engine noise, and between the combustion pressure and the engine noise. The noise coherent with piston impacts was separated from the noise coherent with combustion. Guidelines are presented showing how the results of the coherence model may be used for engine design and noise prediction.