Combustion noise emitted from diesel engines is characterized by short, loud and therefore annoying peak values of the sound pressure which vary from cycle to cycle. These cyclic variations have been observed and investigated for several diesel engines; depending on the combustion system and the operating conditions, the bandwidth of these variations can reach 10 dBA at maximum. Variations in the sound pressure can generally be related to variations in the combustion excitation. Simultaneous acoustical and multi-optical fiber measurements in the combustion chamber of a DI diesel engine showed correlations between combustion noise variations and flame development. Variations within the injection system do not explain the combustion noise variations.