There are two basic sources of noise from a rotor, either main or tail:
1) Pressure noise, caused by its motion thru the air, and determined by its tip speed, the thickness of the airfoil and the planform (the shape, as in swept, square, cranked, etc.) The faster the tip speed and the thicker the blade the more the noise (duh!) In most flyovers, pressure noise is the chief source of noise.
2) Blade-Vortex Interaction (BVI)- Caused when a section of the rotor blade passes thru a spot of disturbed air from another source, usually causing the blade to momentarily get a lot faster, sometimes making Pop sound. BVI is often caused by the preceeding blade, occur mostly in slight descent (vortex drops as it is pushed away from the blade) and occur mostly to blades that are already going pretty fast. On some helos in approach, BVI is the main source of noise.
A Huey is the poster child for BVI noise, since its rotor is going very fast (to squeeze enough lift from the two blades) and the space between the blades is almost perfect to make the Pop at moderate speeds.
Making a low noise helicopter is an art, and costs weight and performance. A slow main rotor is great, but it stalls at low speed, weighs more and doesn't autorotate as well.