Could it be the sound of the slats extending ? At least on the 757's on initial extension there is a brief "groan" that is always heard. I'm guessing as the air gap increases between the leading edge and slat forming a brief harmonic vibration. I also hear it on retraction, kind of reminds me of when the tank in the toilet is approaching full the plumbing gives that slight and brief groaning sound !
That's my opinion though I may not necessarily agree with it.