Cook pasta according to package directions; drain and rinse with cold water. Transfer the pasta to a large bowl; add celery, peas, ham, cheese, pimientos and onion. Toss to combine.
Stir mayonnaise and sour cream together in a small bowl. Add the mayonnaise mixture, salt and pepper to the pasta mixture; stir to coat. Serve immediately, or cover and refrigerate for up to 3 days.
To make ahead: Cover and refrigerate for up to 3 days