Cook the macaroni according to the package directions, opting for slightly overcooked instead of al dente (it is better for pasta salad this way) Drain and rinse under cold water until cool.
In a large bowl, whisk together mayonnaise, milk, grated onion, vinegar, and sugar.
Gently stir in the macaroni, carrot, celery, and green onion until combined. Add salt and pepper to taste.
Cover and refrigerate for 30 minutes. Stir and adjust seasoning before serving.