1. Cook Macaroni: Cook the macaroni according to package directions until al dente. Drain well and rinse with cold water. This stops the cooking process.
2. Prepare Dressing: In a large bowl, whisk together the mayonnaise, milk, sweet pickle relish, white vinegar, sugar, salt, and pepper until smooth.
3. Combine: Add the cooked macaroni to the dressing and mix well to coat. Add grated onion and shredded carrot, if using.
4. Chill: Cover and refrigerate for at least 2 hours, or preferably overnight, to allow the flavors to meld. The salad will thicken as it chills.
5. Adjust Consistency (if needed): Before serving, check the consistency. If the salad is too thick, add a little more milk to reach your desired creaminess. Stir well.