From measured overall I. C. engine noise levels and corresponding measured combustion (cylinder pressure) levels the paper illustrates the major influence that the combustion system has on the engine radiated noise. The basic ‘noisiness’ of normally aspirated and turbocharged two- and four-stroke D.I. systems, normally aspirated I.D.I. (swirl chamber) diesel and gasoline systems are compared by a normalised frequency spectrum method. Using a simple linear model for calculating the direct combustion noise level of each combustion system the relative levels of noise are calculated. The results show good agreement with measured noise levels for normally aspirated two-stroke and four-stroke D.I. engines and indicate that combustion noise is low in turbocharged diesels, I.D.I. diesels and particularly gasoline engines.