Hello all, after a long time of read-only, this topic finally made me write something on this board.
I have been hearing those noises for quite some time, visiting my parents' house in the approach of TXL.
The airplanes pass overhead at approx. 2000-3000ft. The sound can be heard quite often - maybe 5-8 times on some days.
Most noticeable about the sound is that it's usually the first thing you hear of the airplane. It can be heard on both CFM and IAE birds - therefore, I'd say it's definately not engine-related.
At this alt. both landing-lights and spoilers are usually where they are for final, so they're propably not a candidate either.
Somethimes, the pitch does not just go down (WHOOooop) but down-up (-ooooOOOP), so there has to be a parameter-variation that can not just be explained by the Doppler-Effect.
Possible contenders (apart from Spoilers) are:
1) Flaps. (usually don't go out-in)
2) Aerodynamic noise of other components on the airframe.
There's a paper by DLR (German Aerospace Agency) about noise-sources on airplanes.
http://www.fv-leiserverkehr.de/pdf-d...t%20070123.pdf (in German)
Page 55 of the pdf / page 45 of the document will take you to the sound generated by the Tank Overpressure Relief Valves/ Openings.
The tones are in the region of 530Hz and 580Hz at over 110dB, which is quite consistant with the "blowing over a bottle" analogy.
This might be our patient here, with a change of AoA creating different noise-pitches. AoA-changes might be a result of flap-extension over here, so flap-travel might cause the noise on a secondary level.
The frequency of the noise-occurances (5-8 times a day at times) makes me discard the Spoilers as a source, as I have not that frequetly experienced the use of spoilers that low (though I have seen it happen once or twice).