Veeany,
I guess its like trying to describe the taste of a bananna when we describe a slap. The horrendous blade slap of a Huey that can be heard miles away is the transonic sound I describe, the typical lower amplitude popping that many rotor experience is pure BVI, where the rushing change in angle of attack causes some increase in noise as the blade enters the wash of the prevous one. For low disk loading rotors, the vortex does not descend very quickly, and so is more likely to be hit by the next blade.
These pops are more likely to sound in descending flight at moderate speed, and in turns. The sound plots from most rotors have a ""fried egg" shape to them, where the speed and rate of descent conspire to make the high noise boundary, and any faster or slower rate of descent will make the noise disappear.