in a large saucepan, cover the potatoes with water. Bring to a boil over high heat. Reduce heat and simmer, covered, for 20 to 25 minutes or until almost tender, drain well. Cool slightly. Peel potatoes if you like, then cube potatoes. Set aside. Meanwhile in a large bowl, combine the celery, onion and pickles. In a small bowl, whisk together the mayonnaise, sour cream, mustard, sugar, vinegar, salt and pepper. Pour mayonnaise mixture over celery mixture. Toss to coat. Chopegg whites. Carefully fold potatoes and egg whites into celery mixture. Toss to coat. Cover and refrigerate for 4 to 24 hours. Before serving, if necessary, stir in milk to make the salad the consistency you like.