Little Giant pump troubleshooting help

I have a 6cia ml sump pump. I just replaced the diaphram and the switch put everything back together and made sure the little vent tube was clear. I thought it was working properly but now it turns on for about 3 seconds then stops. Almost sounds like a loud hum. Then about 3 minutes later it starts back up for 3 seconds and turns back off. This little interval goes on forever until I disconnect power. I know the pump it self pumps because I tried it before I put it back into the basin..Advise me what to try and thank you.

 

https://www.tractorsupply.com/know-how_farm-ranch_farm-equipment_sump-pump-troubleshooting

 

thanks I was told it was the check valve leaking through. Now it is not doing it so hopefully whatever it was fixed itself.