Sound radiation by a backward facing step under a plane wall jet is examined. The investigation is based on an experiment where the mean velocity, the step heights and the cross-stream extent of the jet are varied. The comparison between the backward facing step flow and the corresponding wall jet shows the existence and some characteristics of a source located at the step. Computations of the sound radiated by the turbulent eddies and existing literature about sound sources near geometric singularities, indicate that the step is involved in the radiation of this source. It is shown that the step diffracts the sound of nearby passing eddies and that the sound level is significantly increased into the upstream directions. Flapping of the separated shear layer does not radiate sound