It is indeed airflow as a result of the flaps running through a certain position. The airline I worked for had the cabin crew include a short spiel about this in their pre takeoff passenger brief.
The BAC 1-11 made a similar racket, though the engines were so noisy on takeoff that the flap howl was only audible on approach at idle thrust.
Last edited by Mach E Avelli; 28th February 2013 at 02:07.