Bring a large pot of salted water to a boil. Cook macaroni according to package directions, until al dente. Rinse with cold water and drain. This stops the cooking process and rinses away extra starch so the noodles will not stick together.
Cool the noodles completely before adding the dressing.
In a large mixing bowl, toss together cooked and cooled noodles onion, celery, bell pepper, and carrots.
In a small mixing bowl whisk together mayonnaise, sour cream, milk, cider vinegar, sugar, dijon mustard, salt, and pepper.
Pour half of the sauce over pasta and toss until everything is well coated. Cover bowl with plastic wrap and chill for at least one hour before serving. Stir in additional sauce before serving. Add a splash of milk, for a thinner sauce, if desired.
Storing Instructions: Leftovers will keep in the fridge for 3 to 5 days.Make-ahead Instructions:assemble the salad ingredients and dressing and store separately in the refrigerator for 3-5 days. Freezing Instructions: Freeze the pasta mixture without the sauce, in a freezer safe container for up to 3 months. Thaw overnight in the fridge before adding the dressing.