Preheat oven to 375°F. In large pot, melt butter over medium high heat. Stir in flour and cook 1 minute. Add milk and cook, stirring until mixture thickens and comes to a boil. Remove from heat and add grated cheese. Fold in mayonnaise. Add macaroni and mix thoroughly. Transfer mixture to 2 qt. shallow casserole dish and place in oven for 30 minutes until hot and bubbly.