Cook elbow macaroni 3 to 4 minutes past al dente, about 10 to 12 minutes total. Drain without rinsing.
Pour apple cider vinegar over hot drained pasta. Toss gently to coat every noodle. Let cool to room temperature completely.
Whisk together mayo, whole milk, sugar, salt, and white pepper until smooth. Taste and adjust seasoning before adding pasta.
Fold cooled pasta, shredded carrots, and grated onion into the dressing using a rubber spatula. Fold gently to keep noodles whole.
Cover tightly and refrigerate for at least 4 hours. Overnight produces the best result.
Before serving, stir thoroughly and fold in 1 to 2 tablespoons of whole milk to loosen the dressing. Taste and re-season if needed. Serve cold.