The buzzing sound you notice is just the P & W engines spooling up, most large planes, 747,767,777 you will notice it more, and also on RR engines it is quite distinct. The bigger the engine, the more buzz sound you will hear, but it is quite distinct on the Airbus aircraft and like I said RR engines...

NW Airbus may have GE engines but I believe they are PW.


Eric