Bring a large pot of salted water to a boil. Add the elbow macaroni and cook according to package instructions until al dente, usually about 8-10 minutes.
Once cooked, drain the macaroni in a colander and rinse under cold water to cool it down. Let it drain thoroughly.
In a large mixing bowl, combine the mayonnaise, sour cream, mustard, salt, and pepper. Mix until well combined.
Add the cooled macaroni to the dressing mixture, followed by the chopped celery, red bell pepper, red onion, and pickles or relish. Stir thoroughly until all ingredients are well coated with the dressing.
Taste and adjust seasoning if necessary. If desired, fold in chopped parsley for added flavor and color.
Cover the salad and refrigerate for at least 30 minutes before serving to allow flavors to meld.
Feel free to customize the recipe by adding other ingredients such as hard-boiled eggs, corn, or cooked bacon for added flavor and texture.
Always taste before serving to adjust seasoning as needed.