Whilst I'm sure the flaps will generate some noise I still think its speedbrake. I live directly under the approach at appoint where the aircraft are through approximately 4500' at, typically, 220 knots and "clean". It's usual at this point to be told to reduce to 180 knots and continue the descent to 3000'. Jets don't "go down AND slow down" ...... so speedbrake is usually deployed and "wooooooooooo". The flap isn't taken until the aircraft are past my house.
Next time I hear one I'll get my binoculars out
and see what speed/altitude it's at with "Planefinder".