Cook macaroni according to package directions. Meanwhile, in a large saucepan, melt butter. Stir in flour until smooth. Gradually add milk. Bring to a boil; cook and stir for 2 minutes or until thickened.
Reduce heat; stir in the sour cream, process cheese, Parmesan cheese, salt, mustard and pepper until smooth and cheese is melted.
Drain macaroni; toss with cheddar cheese. Transfer to a greased 3-qt. baking dish. Stir in cream sauce.
Bake, uncovered, at 350° for 35-40 minutes or until golden brown and bubbly.