I guess many of us on here know of all sorts of aircraft features that change noise when they are operated.
Out here in East London, if you are in say Stratford 146s outbound from London City will retract flaps as they overfly at 3,000', a very distinctive noise when inside the aircraft but also audible from the ground.
And the Heathrow inbounds also change noise considerably as they spool up as they stop descendind, which corresponds to what you hear on board.
It's a good post to ask a point I've noticed from our office car park in Slough looking up at the Heathrow departures. What is it on the IAE powered A321s (eg BMI, Lufthansa) that makes them sound like a buzz-saw sometimes when on climb power?