First thing, no I am not a spotter, enthusiast etc etc... However this phenomenon has had me confused for many years. I have always thought it would be pointless to bring it up since I could never explain it properly.
What I can say is it has nothing to do with configuration changes, thrust changes or IAE,CFM,PW,RR
What I have noticed is it only happens in certain areas (for me in a small radius near where I live in London) and it seems to be only with smaller aircraft like the A320
My conclusions is it must be an acoustic phenomenon, that is dependent on the buildings around you. It's definitely not the aircraft itself but a sound reflecting in a bizarre way.
As I said, very hard to explain but I understand what the op is saying.