Is it a tank with drilled hole with a standpipe or a hang on back (HOB) overflow one?
For HOB, use a valve to slow down the water to the sump and/or make a loop with the tube that's connect between the HOB to the sump so that it always has a puddle of water in the tube/loop.
For standpipe, drill a hole on the very top and stuck a tube into it. Lower the tube would get the water flow faster and hit the water intake and make air sucking noise. Pull the air tube out slowly until the water covers the water intake for a 1/4-1/3" or so.