Cook the Macaroni - Bring a large pot of salted water to a boil. Add the elbow macaroni and cook until soft, following the package instructions. Drain and let cool to room temperature.
Prepare the Dressing - In a bowl, whisk together mayonnaise, milk, apple cider vinegar, granulated sugar, kosher salt, and black pepper until smooth and well combined.
Combine the Ingredients - In a large bowl, mix the cooled macaroni with the grated carrot, celery, and onion.
Mix with Dressing - Pour the dressing over the pasta mixture and stir thoroughly until the macaroni is fully coated and the ingredients are well integrated.
Chill the Salad - Cover the bowl and refrigerate for at least 4-6 hours, or better yet, overnight.
Serve and Adjust - Before serving, give the salad a good stir. Adjust the seasoning if necessary, and add more mayo or milk to moisten if needed.
Notes
Balance the flavors with sugar if too tangy; don't overcook the pasta; grate onions to avoid large pieces; let it rest overnight for better flavor; use fresh ingredients for the best taste.