Many moons ago (zzz) I had the privilege to fly the mighty BV234, and during training we were told that during some oil ops in Alaska/N Slope someone appeared at the heliport to meet a colleague inbound on one of the 234s. The unmistakable slap slap slap duly materilalised and our friend went outside into the minus 20deg cold to watch the arrival. After ten minutes or so the Ops bod got concerned and went out to tell the visitor, who was puzzled that the machine was not yet in sight, that they'd just called in their eta in 40 minutes time (ie nigh on 100 miles), and wouldn't he be more comfortable indoors?
My experience in more temperate climes was that the noise was very variable from arrival to arrival, and seemed to be related to small changes of wind direction or a/c heading - ie the loud stuff seemed very directional. At certain power settings (suspect in cruise-speed descent) the cannon-fire slap was very evident, though the Bell 214ST was capable of just as much, if not more in certain regimes of descent.
Again, huge broad blades slapping thru the vortices.
But Chinooks not audible seconds away? Hmm.
Around this part of the world they are clearly audible about five times earlier than the Apaches, Merlins and occasional Sea Kings that follow the ridgeline I live on.
Bless'em!