Cook macaroni according to package directions. Pour into a colinder and rinse with cold water. Allow to drain well and cool for 15 minutes.

In a bowl, whisk together mayo, milk, vinergar and sugar. Add the macaroni,  carrots, and onion to the mayo mixture and fold together until evenly coated. Add salt and pepper to taste.

Cover and refrigerate at least 4 hours (best if overnight). Gently stir before serving adding a little more milk if needed to thin, but no more than a tablespoon or two.