Combustor Rumble can be a problem in engines that reverse the combustion gas flow prior to entering the 1st stage gas producer nozzle. Audible Combustor Rumble is created by unstable combustion, in addition to creating a significant increase in engine vibration during combustion instability. Unstable combustion occurs when the Air/Fuel ration is less than 50 to 1. It is similar to the audible rumble heard when adjusting the air quantity on an oil furnace. The air quantity is