In a large pot, bring salted water to a boil. Add the elbow macaroni and cook it according to the package instructions until it's tender. This should take about 8-10 minutes.
While the macaroni is cooking, shred the carrots and onion, set aside.
Once the macaroni is cooked, drain it well and transfer to a large mixing bowl. While the macaroni is still hot, add the apple cider vinegar, shredded carrots and onions. Combine well but be careful not to smush the noodles as you stir, I usually fold the ingredients together. Cool in the fridge for 20-30 minutes or until completely cooled.
While the macaroni is cooling, in a separate bowl, combine the mayonnaise, milk, sugar and salt and pepper to taste.
Pour the dressing over the cooled macaroni and combine thoroughly. Taste and season with more salt and pepper, adjusting to your liking.
You can serve immediately but I prefer to cover the macaroni salad and refrigerate for at least 30 minutes. This allows the macaroni to absorb some of the dressing and the flavors to meld together.
Just before serving, check the salad. The macaroni likely would have absorbed a lot of the dressing. If it looks a bit dry, just add a splash of milk and stir until it's as creamy as you like it.