A simple technique has been developed for measuring the noise radiated by diesel engine surfaces in response to combustion excitation. Results using this technique correlate well with the established computer-based analysis technique. Combustion noise measurements from a direct injection engine show increases of 3dB(A) as fuel cetane number is reduced from 50 to 40CN. Combustion noise in high speed direct injection engines is much greater than in comparable indirect injection engines, and a fundamental study is described which indicates the rate of heat release diagrams required to minimise and optimise combustion noise. This work is supported by engine results with pilot injection which gave reductions of 5 to 8dB(A).