All toilets do this, but with the plastic adapter, you can hear it a little more. Nothing to worry about. As the bowl gets filled up, it's like a cup of water that is being overfilled. It takes it a moment to settle down as you stop adding water to it as it dribbles over the edge. You'll notice it, too, when you are sitting there and adding things to the bowl...the bowl is filled just to overflowing, anything that is added causes it to overflow.