Cook macaroni and shrimp in salted boiling water until pasta is just tender and shrimp are pink, about 8 to 10 minutes total. Drain and rinse with cold water until cool.

Whisk together mayonnaise, sour cream, Dijon, lemon juice, Old Bay, salt, and pepper.

Fold pasta, shrimp, celery, bell pepper, and green onions into the dressing until evenly coated.

Cover and refrigerate at least 1 hour. Stir again and add a little mayo or milk if the salad seems dry.

Spoon into a serving bowl, top with fresh dill and black pepper, and serve cold.