The lighthouse keeper was a rotorwing enthusiast and had done some light a/c flying, as I understand. He seemed quite sure that there was no change in sound, that they went straight in.
Although I have a son who flies them, I am not particularly an enthusiast of rotorwings and have done quite a lot of light a/c flying. What I am very familiar with is the sound of Chinooks approaching and passing very low overhead my house, especially late at night. (I love it!!). I would defy anyone who was not an expert to draw any conclusions from the way the sound changes as the aircraft approaches even when in a steady cruise.