It is the main pump. So when this happened the pumps were not on, but of course their is alway water moving through the filters. It just started. (Maybe someone hit the pump with their arm and turned it on. When I say pulsating it sounds like it is on, off, on, off every two seconds. So what do I need to do. My filters probably need cleaning? I am not only a newbie here, but I am a spa newbie. My husband is very handy though!