I assume it is an AC non controllable model? Unlike an impeller which can spin either way and pump, a prop can only work one way, on start up if it spins the wrong way which it usually will as that is the path of least resistance, it impacts the front cover and falls back spinning the right way and that is what makes the noise. Generally, if you cycle them off and on with 15 minute delay, the propeller will last at least 2 years but very fast cycling in seconds on a wave maker and the impacts will brake it within a couple months. The DC pumps use electronics to guide the start so they don't make this noise.
