This paper deals with the study of the structural response of an operating diesel engine. The linear relationship assessed in operating conditions between the cylinder pressures and the noise is called spectrofilter, and is commonly used to separate combustion and mechanical contributions from the overall engine noise, using Wiener filtering. A first part of the paper proves the necessity to remove the deterministic part of signals in the identification of spectrofilters, using indirect criterions as the causality of identified filters and their stability to changes in operating conditions. A second part concerns the comparison between identified filters and results of impact hammer tests, where it is shown that spectrofilters can provide useful information about the dynamic behaviour of the engine block.