sump pump outflow suction

I'd guess it's a syphon effect, have you a vent pipe on the outlet anywhere ?

If the pipe goes up and then down again you may need a breather at the top of the loop to break the suction.

If the check vale above the pump is OK check for leaks on the rising delivery pipe.

An air valve at the highest point may also help.

Is downstream the pump or outflow side?

Sketch the system and send it along. Include heights of various points compared to each other, and also complete the entire story such as:

1. Was it fine, now not ?

2. What has changed ?

3. Did you check the impeller to see if it 'sanded' and now smaller or eaten away, thus not really pumping to the end of your outflow pipe and it all comes washing back ?

4. {{ Force yourself to list 3-4 other facts or comments even if you think they are not important.}}

. . . . waiting !

I am a new occupant of the house, so I don't know how it was before. We've recently had a lot of snow melt and so the pump was running alot. There was a leak in the outflow pipe between the pump and the check valve. Since performing the changes I mentioned, the pump runs less frequently (from every 2 minutes to about every 10-15 minutes). I do believe the problem was the siphon effect. I did not seal the joints in the new outflow pipe and I can hear sucking. I don't think I have the height for an air vent, but I may install the check valve in the "water hammer" extension to allow air to be drawn in and keep the water in the pipe. Thank you everyone for your help! I'll keep you posted. Happy New Year!