The blade tips move quite fast, often more than half of speed of sound.

In cruise flight additionally the advancing blade moves faster relative to air than the retreating one, so even the generated sound changes as the rotor turns.

This effect increases as the helicopter accelerates. If it overspeeds, blade tips on the advancing side may (depending on helicopter type) get close to the speed of sound and shockwaves start to form on that side that add even more pulsating sound.

In some cases (turns at high speed, descent) the blades may also be hitting the wake vortex shed by the previous blade resulting in sharp increase in the puslating sound called "blade slapping". The reason is the blades only hits the vortex when it passes one particular place on the rotor disk, usually on the advancing side.

This effect would make the flying helicopter sound much more immersive!!