The fill valve isn't adjusted correctly. Does it have a ballcock or a newer fill valve? If it's the ballcock, there's a screw in the middle of the fill valve that you can loosen to lower the ballcock. If it's newer, you'll see a float that surrounds the fill valve. It has a small metal clip toward the bottom that you need to pinch to raise or lower the float. It also has a large plastic screw with a phillips head at the top for fine tuning.
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The fill valve isn't adjusted correctly. Does it have a ballcock or a newer fill valve? If it's the ballcock, there's a screw in the middle of the fill valve that you can loosen to lower the ballcock. If it's newer, you'll see a float that surrounds the fill valve. It has a small metal clip toward the bottom that you need to pinch to raise or lower the float. It also has a large plastic screw with a phillips head at the top for fine tuning.