Remove tips and strings from pea pods. Cook macaroni according to package directions in lightly salted boiling water, adding pea pods and peas during last 1 minute of cooking. Drain and rinse. Halve pea pods diagonally; set pasta and pea pods aside.
In a small bowl stir together mayonnaise, sour cream, milk, mustard, garlic, salt, and pepper; set aside.
In a large bowl combine the cooked macaroni and pea mixture, celery, and onion. Pour mayonnaise mixture over macaroni mixture. Stir gently to combine.
Cover; chill 4 to 24 hours. Stir mixture before serving. If desired, add milk, (1 to 2 tablespoons) to moisten. Top with additional pea pods, if desired. Makes 12 to 16 side-dish servings.