What is this very distinct humming noise coming from the IAE-V2500 engines of the A320 during taxi?
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.