I use to work for NWA, and commuted to work on this plane all the time. It is a great plane. That noise you hear is the flaps retracting. It's sort of high pitched tone. There is a similar noise when the flaps extend for landing. That one is more a deeper sound, and doesn't last quite as long. My friend that worked there, said they refered to the noise when the flaps extended as "Jurrasic Park", because it sort of sounded like a dinosaur.