Bring a large pot of salted water to a boil. Add 1 pound elbow macaroni and cook according to the package instructions for al dente, about 8 to 10 minutes. Once cooked, drain and transfer to a large mixing bowl.
Meanwhile, grate the 2 carrots and an onion to get ¼ cup grated yellow onion, set aside.
While the macaroni is still hot, add the 2 tablespoons apple cider vinegar along with the shredded carrots and onion. Stir to combine then chill in the refrigerator for 20 to 30 minutes or until completely cooled.
While the macaroni is chilling, combine the 3 cups mayonnaise, ⅓ cup milk, and 1½ teaspoons granulated sugarsaltn a separate mixing bowl.
Pour the dressing over the cooled macaroni and combine thoroughly. Add salt and ground black pepper to taste.
You can serve immediately but I recommend chilling for an additional 30 minutes. This allows the macaroni to absorb some of the dressing and the flavors to meld together.