Never found the reason until now. The ugly disgraceful whine sound of A320 family. Only at landing. But not anymore before touchdown. Thanks a bunch they found a solution. Unfortunately still some aren't fixed.
The poster above mentioned diagrams. I share them. The frequencies of those different sized openings are stated as 530Hz and 580Hz. Absolutely disharmonic and annoying, See here the recording of an A320 overfly. Sonogram: Left to right is approaching over leaving the microphone position. More black means higher noise level. See the 2 strong curves, that's the dominating whine sound. And it is the first noise one hears and the last before the noise disappears. Due to doppler effect the frequency lowers during overpass.
