What is humming in combustion chamber?

Due to turbulence of incoming air and/or fuel mass flow perturbations the heat release process will be periodic (Unsteady) so the pressure inside the combustor will fluctuate due to unsteady heat release rate causing broadband noise this noise will match with one or more of the acoustic modes (frequencies) of the combustor, then the humming will occur and if the humming reflected from the combustor boundaries to the flame again it will increase the root source of the problem (flow perturbations) and amplification of the oscillation will take place causing structural vibrations to the combustor walls (acceleration).