Well,
Used to be able to hear them very loud for minutes before they came over at night (not so during the daytime due to the noise outside anyway) but at night for sure...
Infact one came past the other night very low but could barely hear it even with the wind coming from that direction. Within seconds of it passing the sound was gone, which is again unusual. Can usually hear it just as long with it retreating!
Must be atmospherics? As i can't recall any of the Chinooks receiving such upgrades to make them quieter.