Can you post a close up photo of the venturi? I'm not sure what would cause what you're talking about, but remember that a venturi will work best with a particular range of water movement through it and especially with a steady flow. It's all about how much speed the water picks up in the venturi area, and consequently how much of a pressure drop is created. So your pump may have something to do with the burping. A typical scenario is that air enters the water at the venturi, creating foam, which means the water column expands in volume right downstream from the venturi, which causes friction, and with a weak pump, a slow down of the water, which can cause the foam to "back up" a little in the venturi area and interrupt the flow of air into the water stream. So, this causes less foam, which lowers the line friction, it speeds up, and the whole cycle starts over. Does this sound like someting that might be going on?