I think that's A320 hydraulics noise thread No. 1000
The other regular one, alas less now than previously, is the BAe146 flap extension/retraction howl. Apparently when the prototype first flew this noise was quite a surprise. It's a harmonic that develops at certain airflows between the inner flap edge and the wing root. After many comments BAe developed a modification to overcome it, but it weighed 100kg, two passengers, and as far as I know no operator took it (except maybe the Royal Flight aircraft).
I'm surprised Airbus haven't developed a mod for the hydraulic noise.