A number of experiments have been performed on the properties of propane diffusion flames at relatively low fuel flow rates and using a variety of burner types. Optical methods were used to observe the flame and plume above it. We have studied the transition from a steady flame, with a plume exhibiting a helical instability at low flow rates, to an axisymmetric instability of the whole flame/plume, that grows from the base of the flame and oscillates at a well-defined and robust frequency, at higher flow rates. These results and the observation of the effects of various external modifications, e.g. the generation of an annular counter flow, a pressure perturbation, etc. are consistent with the view that the transition to the axisymmetric state, at which the flame flickers, is one to a globally excited oscillation forced by a region of absolutely unstable flow at or near the base of the burners used in this study.