It seems to occur when throttles are pulled back, or perhaps on some change in configuration and takes the form of a single, quite loud groan
I live on the approach to IAD and have heard that same noise many times and wondered what it is/was?
To be honest the sound from the ground does sound like an extended "groan" with the pitch decreasing through the duration of the sound.
Clues:
- Always occurs on approach
- Only heard this with twin engine aircraft (wing mounted, A320/737...?)
- Not heard with smaller RJs (at least I don't believe so)
My house is 8.75 miles to 19C, so that distance may give a clue
- GY