(i) forming, coalescence and breaking of bubbles, (ii) bubble eruption at the interface, (iii) passage of bubbles (iv) interactions between fluidized particles and (v) gas-phase oscillations in plenum chamber. Pressure fluctuations due to gas-phase turbulence in plenum occur for distributors of low resistance while those due to particle interactions are dominant very close to the distributor plate . In the current study, we may conclude that most of the oscillations arise due to factors (i), (ii) and (iii) listed above.