As the slat extends, airflow is directed to the space behind the slat, rather than the bottom of the wing...creating what they call as 'cavitation resonance'.
I find it similar to blowing across the top of a bottle.
That's the noise! I opened a thread about two years ago, asking almost the same question as the OP, although from a rather different viewpoint. I live in south London, where you guys are descending to intercept the ILS on 28L at Heathrow. I used to hear the 'blowing over a bottle neck' noise - I thought of it as a 'sharp groan', or 'dropping out of warp' - quite frequently, but not as often of late.
Of course, you will be at or a bit below 5000ft over my house, so aeroplanes would be in quite a different configuration to the OP's 18000, which is a bit of a mystery. My nephew is a Boeing driver and hadn't got a clue what I was talking about, so maybe it is only buses?
Roger.