The transition from low-Amplitude broadband noise generation to large-Amplitude discrete tones due to combustion instability as the flow conditions are gradually varied is experimentally investigated in a half-dump combustor. The length of the combustor duct and the axial location of the step relative the acoustic mode excited in the duct are independently varied. High-Amplitude discrete tones are excited under certain conditions when the observed dominant frequency jumps from that of a natural acoustic mode to a vortex shedding mode. This condition is denoted as the onset of combustion instability, and an experimentally determined stability map is obtained of the conditions of the onset of instability. High-speed imaging of the chemiluminescence fluctuations in the flame stabilization zone is performed at different conditions such as far away from onset of instability and after at the onset. Fourier transform of the time variation of the fluctuation in the total chemiluminescent intensity in the combustion zone is performed, and the resulting spectra is correlated with the acoustic spectra observed at the base of the dump plane in the duct. It is observed that the total chemiluminescence spectra show distinct peaks at exactly the same frequencies as the dominant frequencies of the corresponding acoustic spectra, clearly indicating that the heat release fluctuations in the combustion zone are responsible for the excitation of the acoustic oscillations. Further, the different dominant frequencies observed in both spectra can be identified as corresponding to either a vortex shedding mode or an acoustic mode, depending upon the flow conditions relative to the onset of instability, so the results show that the acoustic modes excited in the duct in turn influence the vortex shedding process involved in the heat release fluctuations.© (2006) by the International Institute of Acoustics & Vibration.