Waste lines from a sewage ejector or sump pump use a check valve to stop wastewater from flowing back into the basin. When these valves shut, they can make a thud loud enough to be heard throughout the house.
To quiet them, replace the old check valve with a “silent” or “soft close” check valve.
Remove the section of waste line under the check valve, disconnecting it at the union or coupling above the basin—or cutting the pipe.
Drain the water above the check valve, then loosen the clamps and remove it (Photo 1). If you don't have a shutoff valve above the check valve, install a “full port” shutoff valve on the waste line above the check valve. You won't get soaked with dirty water the next time you service the check valve.
Cut the waste pipe to the proper length and attach the silent check valve (Photo 2). Reassemble the pipes, open the shutoff valve and turn on the pump.
Silent check valves are available from most plumbing suppliers. If you have trouble locating one locally, search the internet for silent or soft close check valves.