I'm having a very strange problem with my newly installed sump pump and I home someone here can maybe offer some advice.
The pump works fine as far as removing water, but right after the pump shuts off, I get a repeating thumping noise that increases with frequency. This thing is under our master bedroom and it's driving us nuts! I am guessing I have some sort of pressure wave/fluctuation in the outlet pipe that is making the check valve flutter, but I can't think of what to do to fix it. I thought about removing the check valve but the water in the pipe fills the pit back up almost all the way when it rushes back in. It's a 1/2hp Simer pump pumping through about 70 feet of 1.5" PVC with only about 3 feet of head. It discharges to a low spot in our backyard.
I made a video of the problem so you can hear what it's doing with captions to explain. Video is here:
The splashing the water makes is kind of loud, but if you listen closely you can hear the thumping. From upstairs the thumping is much louder than the splashing.
It is the check valve flap bouncing. Once it starts it is like a coin spinning down, the noise gets faster and faster until it quits. you need a different type of check valve or a different location for the one you have.