Dar is correct about the ground effect, it reduces the speed the air will go through the helicoptor and on reason they will often fly to higher altitudes than they can take off from. I may be wrong but a prop or wing create no lift when stalled, you are talking about a near stalled condition. The scream from the prop may be the blades cutting near or above mach 1, you can hear it when tangent to the prop but the volume goes down greatly as you go away from tangent. Though being near stalled condition would be less efficient I'm not sure this makes more sound. Seems like on varible pitched fans there is no additional sound when stalled, but they operate at much slower speeds.