It sounds like one of two problems -- neither of which is severe. First, check to make sure the flapper in the tank is closing completely after the flush and seating properly against the opening of the flush valve at the bottom center of the tank. If that's not the problem, then you should replace the fill valve in the tank. The Ultramax is equipped new with a fill valve made by Korky and I'm told they are readily available in hardware stores. If you can't find the Korky, you can use a Fluidmaster. They're inexpensive and easy to install. Just follow the directions that come with it.