from my experience the moo cow noise seems to always be at low N1 rpm when the initial light off occurs on any turbine engine. On the 777 fuel is automatically sequenced by the EEC, but even on previous jets and turbo props, it seemed to make little or no difference to the low growling noise during initial light off if you let the starter spin faster before introducing fuel. It was usually evident in the cases when you had to let the starter spin to get the ITT/EGT down before whacking the fuel in on a quick turnaround. Though I do remember it used to stop the 'boom' compared to introducing fuel at close to minimum light off N1.