my inlet hose goes into the water too, and thats what causes the slurping from above or at least makes it worse. If the hose is above the water line in the sump, then ait can travel in the opposite direction of water and the pressures would remain equal, but that means hearing the sound of of water plop into the sump (unless you get a nice drip plate)
... thats why I think the easier solution is threating the tubing. 1/4 inch may even be more than you need. and when you thread it, make sure it does NOT reach the bottom. the ideal end point is different for every setup so just play with it. you may need only 6 inches of tubing just at the top, or more, reaching close (but never into) the sump.
hope that helps, let me know how it works out
joe