It's the combustor organ-pipe resonance, isn't it? And I'm amazed the EPA didn't make them control/capture all that unburned fuel!
It strikes me that if it were unburned fuel there would have been many incidents of toasted aft fuselages on L1011's. if it were to light off externally
I'll go with typical steam and smoke out of the exhaust until the smokeless combustor does its thing.
Any old hands on here like 411 and rampers who know?