It took me some time being around the RJ
's at work this week, but I think I have the 'mystery' noise figured out. I only hear the noise when the aircraft is taxing from my left to my right, and with that in mind....and with the APU Intake on the RH
side, I think it comes actually from noise from the the APU Intake being muttled from the engine exhaust........Kind of muttled sound.
"It's not the size of the dog in the fight, but the size of the fight in the dog"