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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.