Meanwhile, preheat oven to 350°. In a large saucepan, melt butter. Stir in flour until smooth. Gradually add 1-1/2 cups milk. Bring to a boil; cook and stir 2 minutes or until thickened. Reduce heat; stir in sour cream, process cheese, Parmesan cheese, salt, mustard and pepper until smooth and cheese is melted, adding additional milk to reach desired consistency.
Drain macaroni; toss with cheddar cheese. Transfer to a greased 3-qt. baking dish. Stir in cream sauce.
Bake, uncovered, 35-40 minutes or until golden brown and bubbly.