Unstable combustion...... during light-off, the flame front in the combustion chamber starts from the ignitor plug and tries to spread out. At low rpm's during the start cycle, the air flow and fuel pressure are low enough that the flame front sort of bounces around within the combustor and cant establish itself. Thats the low frequency rumble heard during the start. As rpm increases, higher air flow and better spray patterns from the burners allow the flame front to settle, and the start cycle smooths out to stabilised idle.