On helicopters the sound is called blade slap. Assuming the blades are evenly spaced and youre standing still, the rate, or frequency at which a given blade is moving toward you is constant.
As a blade moves through the air, it forces the air out of the way, creating a pressure front.
It is this pressure, reinforced by each blade, that you perceive as noise and which generates the chopping sound.
If it should get quiter over time, it would only do so if the airfoil changed its structure, and since plastic is harder than air, or you dont fly in sandstorms, there is very unlikely thats going to change