In start-up and hover each blade produces more or less constant sound. But the sound is attenuated by distance and may not be the same in all directions. Therefore you hear it differently depending on the blade's position relative to you. So as the blades rotate, the sound you hear pulsates because the blades alternately get to positions where you hear them more or less strongly. In this video showing helicopter start-up from cockpit you can clearly hear the swish of each blade as it passes overhead with the pulsing increasing in frequency as the rotor spins up.
The blade tips also move quite fast, often more than half of speed of sound, so Doppler effect is adding more variation to the sound if you are standing to the side.
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. Apparently it is rather complex; I found there is a paper about it (not read it; it is behind paywall).