What is this very distinct humming noise coming from the IAE-V2500 engines of the A320 during taxi? So I am a huge fan of the A320, and in a less broader sense, the A320 with IAE-V2500 engines. Arguably the greatest engines in the industry with the greatest sound. However though, I've had a question that I just can't seem to have answered so I'll ask here. If you're sitting inside an A320 with IAE-V2500's and you're taxing out to the runway, you here a very distinct humming noise coming from the engines. What I find confusing about this noise is that it sounds very different on the outside as I have spotted the A320 with V2500's in person, and the idle/taxi sound outside of the aircraft sounds very different from the inside. You can find the sound I'm talking about at timeframe 7:42 until takeoff (taxi) in this provided YouTube video: 
                
             

Thank you all so much! -Niko
I don't think there's anything particularly special causing that sound, it's just how the engines sound.

Inside the airplane you're heavily insulated from exterior sounds, so you're not going to hear much of the airflow and fan sounds from the engine.  You're mostly hearing engine sound that travels through the airframe itself, the turbines primarily.

Outside, the engine sound is dominated by the airflow and fan noise.