Most helicopters are turbine driven, so whats the pop pop popping noise..A friend at Erickson Air Crane says its not the blades passing through the air but the tips of the blades breaking the sound barrier. Is that correct?

Briefly explaining, the rotor blades are running into their own air wakes. The sound occurs more noticicably because of the direction the aircraft is flying--upwind, downwind, etc.

I think it's just the sound of the air being moved out of the way by the blades, as they cannot move as fast as sound can. This is why heavy two bladed helicopters make more noise than four blades, they have to move more air at one time.

Hate to say it, Tam, but your answer contradicts itself in saying that the sound comes from air being moved out of the way, yet helicopter rotor blades can't exceed the sound barrier. The only time a noise like that will happen is when the blade or wing does break the sound barrier. I'm not a technical guy, but I do have a few thousand hours in flying rotary wing aircraft, and I can tell you that the physics of flying involve both Newton's and Bernoulli's principles, and the waves created to induce flight are the ones that are being hit by the trailing blade, creating that smacking noise.

OK robboy.
1: If they even thought about going anywhere close to the speed of sound, they would fly off!

2: Open your hand, and then wave it quickly past your ear. This is not going at the speed of sound, but sound 'waves' are heard. This is similar to a rotor blade but on a bigger scale. This is why a two rotor makes more noise than a four rotor.

OK, by now I'd have thought some physics egg-head might have weighed in here, but in the meantime I'll see what I can do with my pitifully deflated can of physics whoop-ass.
Far as I know, that sound is peculiar only to helicopters, but if I were able to come right behind that hand that was whooshing by my ear with another hand, I'd hear a mini-version of that popping noise. Provided, of course, that both hands were shaped like an airfoil. You'll get a noise from anything moving through air; a golf club, tennis racket, ruffled skirt, etc, but you won't hear that shock effect that rotor blades make, unless you follow it up with something running (VERY quickly) into the disturbed air that's been created right in front. As to multi-bladed aircraft making less noisy wop-wops, you're right on target; the blades are making smaller waves, and the ensuing blade is traveling a shorter distance before making contact.
Earlier attempts at making helicopters exceed the sound barrier resulted in blade(and sometimes airframe structure) disintegration, mainly because designers simply forgot that the blades are turning faster than the fuselage is being propelled. They don't just fly off.

Robboy,
aight, I'm about 5 years late on this, but it IS from the blade tips breaking the sound barrier. It has to do with the angular velocity of the tips at any given time being at an instantaneous velocity high enough to "snap" the air. Think of it as a whip: if you flick a whip, you won't get it anywhere near the soundbarrier, but the tip (if you crack it) will snap. Same Principle.

The reason why you don't hear it when they're flying away is because of the dopler effect. As the heli is flying towards you, the sound experiences a "blue shift", or compression of the soundwaves as they pile up on each other and when it's flying away, it experiences "red shift" as the soundwaves aren't being forced in to each other since they have room to expand. (see also firetruck/ambulance/police car).