But in descent, thrust is at idle (normally) - especially if the speedbrakes are out! (you don't put power against drag) - so I dont think it is engine noise.
I fly the A319 and have heard this from inside the aircraft - it is usually upon speedbrakes retraction - so I think it's purely an aerodynamic phenomenon. If the brake are out at hi-speed (>320 knots) the speedbrake retraction takes 30 seconds so it may prolong (or reduce?) the effect.