The acoustic instability of a partially premixed methane flame is studied in a backward facing step combustor. The bifurcation is performed for constant equivalence ratio of 0.8 by varying the air and fuel flow together. The combustor has two regimes namely vortex mode with linearly varying frequency and acoustic mode with a constant frequency. Further, the zone of linearly varying frequency results in a constant Strouhal number, St ∼ 0.2 conforming the presence of vortex mode while the acoustic modes are characterized by the constant frequency zones as the flow rates are varied. The mode transitions occur at 1000 lpm (Re = 18000) of air flow with a steep rise in the amplitude. Such mode shift occurs when the vortex shedding frequency approaches half of the duct acoustic mode. A simultaneous time resolved PIV, CH∗ chemiluminescence measured for two cases of flow rate 600 lpm (Re = 11100) and 1200 lpm (Re = 22200) in the linear and constant frequency zone respectively. The velocity spectra showed dominant vortex shedding frequency for 600 lpm and dominant acoustic frequency for 1200 lpm. The spectra of velocity fluctuations shows flapping mode to be dominant near the reattachment point. The vortex roll up was found to commence at the step corner when the pressure amplitude changes its sign. Three distinct modes are found for 1200 lpm case, which, are the acoustic mode, nonlinear low frequency mode and the low amplitude mode, whereas only vortex mode was prevalent for 600 lpm. The flowflame field for the acoustic mode showed an intense vortex growth with combustion within the vortex. The nonlinear mode showed ordered structures but lesser vortex amplification. But the flow field in the low amplitude modes consisted of the Kelvin Helmholtz instability of the shear layer with disorganized structures and with heat release fluctuations.