Boil the macaroni: Bring a large pot of salted water to a boil. Add the macaroni and cook 2–3 minutes longer than the package suggests, until very soft. Drain but do not rinse.
Add vinegar: While the pasta is still warm, transfer it to a mixing bowl and toss with the apple cider vinegar. Let it cool to room temperature.
Make the dressing: In another bowl, whisk together mayonnaise, milk, sugar, salt, and pepper until smooth.
Combine: Once the pasta has cooled, pour the dressing over and mix gently. Fold in grated carrot and onion if using.
Chill: Cover and refrigerate for at least 4 hours or overnight for best flavor.
Serve: Stir before serving. Add a splash of milk or mayo to refresh if needed.