Cook noodles according to package directions, then drain and rinse under cold water until completely cooled. Toss with a teaspoon of oil if needed to prevent clumping.
In a large bowl, combine mayonnaise, soy sauce, rice vinegar, and sesame oil; whisk until smooth and creamy.
Add cooled noodles, shredded carrots, diced cucumber, and sliced green onions to the dressing; toss gently but thoroughly to coat all ingredients evenly.
Season with salt and pepper to taste, then sprinkle with toasted sesame seeds.
Transfer to a serving bowl, cover, and refrigerate for at least 20 minutes before serving to allow flavors to develop.
Just before serving, gently toss the salad again and taste for seasoning. Add more mayonnaise if it seems dry, or a splash of rice vinegar if you want more tanginess.
Garnish with additional green onions and sesame seeds before serving.
