For the dressing: Whisk the mayonnaise, sour cream, mustard, vinegar and sugar together in a small bowl or glass measuring cup. Season with salt and pepper.
For the pasta: In a large mixing bowl, combine the macaroni, dill, celery, bell pepper and red onion. Drizzle the dressing over the salad and toss to combine. Season with salt and pepper. Serve immediately or keep up to 2 days in the refrigerator until ready to serve.