There is now a growing interest in every means of reducing the noise of current diesel engines and designing even quieter engines for the future, perhaps as awareness of noise as a social problem becomes widespread. This paper discusses the production of engine noise, its transmission through the structure, and its subsequent radiation as a sequence of events.Diesel engine noise can originate from several mechanical sources such as piston slap, timing gear rattle, etc. as well as from the sharp rise in cylinder pressure which attends combustion. The relative importance of these sources is affected by clearances between the working parts which are dictated by design and manufacturing constraints. If reduction of noise at source is contemplated, it is necessary to diagnose which of these sources are primarily responsible for the noise, bearing in mind that the relative importance of the sources varies with engine load and speed. The diagnosis of sources can be based upon measurements of combustion spectra at various engine conditions.Engine noise may be reduced either at source or by reducing the vibration of the external surfaces of the engine. If reduction of noise by treatment of the surface panels and covers is contemplated, it is generally necessary to treat all the thin section areas (for example, crankcase, water jacket, sump, valve gear covers, etc.). However it is useful to be able to locate any areas which do not contribute significantly to the overall noise. Techniques to find the important noise-radiating areas of the engine surface are discussed prior to a brief consideration of noise reduction measures.