Combustion instability is characterized by large amplitude pressure oscillations driven by unsteady heat release from chemical reactions. Experiments were carried out in a cone stabilized rectangular chamber burning liquid petroleum gas (LPG) with air at Reynolds number between 4993 and 9487. The influence of variation in inlet length on pressure perturbations of the cone stabilized flame was determined. While the cold flow measurements in the combustion chamber showed influence of inlet length on the characteristic standing wave frequencies of pressure oscillations in the chamber, the heat release from the diffusion flame did not respond to the standing wave frequencies. The frequencies of heat release, obtained for different inlet lengths, correlated well with the variations of the cold flow velocities measured in the combustion chamber for different inlet lengths. Combustion instability is seen to be driven by low frequency oscillations from the vortex shedding behind the conical flame holder rather than the standing wave modes. The standing wave modes of the combustor are responsible for the onset of high frequency combustion oscillations and a mode switch occurs at certain inlet pipe length. © 2011 Elsevier Ltd.