As Shawn said, since prop pumps have to correct direction, they only propel water forward when spinning the right way, their are 2 options, electronic pumps like the 6055 that use a sensor and electronics to guide start up and don't click as a result, and synchronous pumps like the 6015, 6025 and 6045 which instead use a mechanical stop. This mechanical stop clicks as the propeller hits the stops, they cannot be used on wavemakers. This clicking is caused by the propeller impacting the front cover and changing direction. This will occur on every start and start cycles of more than once every 15 minutes will void the warranty and lead to propeller breakage.