The blade slapping noise is caused by the a blade hitting the wake
turbulence from
the blade before it. The effect is amplified when you're descending between
50 and 80 knots. I use it to tell my wife I'm on my way home as I fly over
the house.

In an OGE hover you can actually use the sound to tell you if your altitude
is
changing. The system is the most quiet when you're altitude is the totally
stable, and
any downward change causes a pretty dramatic increase in the sound.

Bart