In a large bowl combine the macaroni, celery, onion, parsley, and tomato, if using. In a small bowl, whisk together the Mayonaise, mustard, sugar, vinegar, sour cream, and salt. Pour the dressing ove the salad and stir to combine. Season with Salt and Pepper to taste. Serve.
Store covered, in the refrigerator for up to 3 days.