Dont know the reasons why but I recall when I was on the B737-300 with Orion Aiways. We were (I believe) the launch customer and we had "Jean-Claude" on the flight deck from CFM who was with us to observe engine observation. We had noticed this "noise" when the thrust levers were retarded for descent. We asked him what caused it and (imagine this in a French accent) he said "Of course - that is rumble!".