Cook the elbow macaroni according to package directions, then drain and rinse with cold water to stop cooking. Keep rinsing until the pasta feels cool to the touch.

Whisk together mayonnaise, sour cream, milk, white vinegar, sugar, Dijon mustard, salt, and pepper until smooth. Whisk for about 1 minute so the dressing looks glossy and fully combined.

Combine the cooled pasta, diced ham, cubed cheddar cheese, thawed peas, diced celery, diced red bell pepper, and finely diced red onion in a large bowl. Toss gently just to distribute the vegetables and cheese.

Pour the dressing over the salad and toss until everything is evenly coated. Scrape the bottom of the bowl so no dry pasta is left.

Refrigerate for at least 3 hours, or overnight, to let flavors meld. Chill until the salad is firm and cold throughout.

Stir before serving and add a splash of milk if needed to loosen the dressing. If it looks too thick, drizzle in milk 1 tsp at a time until creamy and spreadable.