In a pot of boiling salted water, cook the macaroni according to package directions (see notes). Drain and transfer to a mixing bowl.
Sprinkle the apple cider vinegar over the top, add the carrot and onion, stir, and let it cool slightly, about 10 to 15 minutes.
In a small bowl, whisk together the mayonnaise, milk, and sugar.
Stir the mayonnaise mixture into the macaroni, and season with salt and pepper to taste.
Cover and refrigerate for at least 1 hour or overnight.
Gently stir before serving, and if needed thin it out with a teaspoon or two of milk or a little more mayo.
