Pressure perturbations arise in combustors from direct noise related to the change in density in flames, as well as the indirect (entropic) noise associated with the acceleration of non-homogeneous regions of flow through nozzles. In this paper we review our recent work on quantifying the relative contributions of direct and indirect noise generated from perturbations in temperature and composition, and the resulting transmitted and reflected pressure perturbations. We show that (a) isentropic models are inadequate to capture the acoustic and entropic transfer functions across a nozzle; (b) corrections to non-isentropic behaviour are possible using existing models for orifices using a single parameter accounting for losses; (c) the behaviour of low frequency entropic noise generated in a chamber can be entirely accounted for when reverberation is taken into account; and (d) indirect noise due to compositional fluctuations can be as large as entropic noise arising from temperature fluctuations. The findings have implications for both the study of entropy noise in model systems, as well as for understanding how to separate the origins of noise in practical systems. In particular, the role of compositional noise in gas turbines (regarding for example the role of cooling in rich-quench-lean turbines) and the role of non-isentropic effects is not accounted for in current models, and should be revisited in the light of current findings.