Melt 1/2 cup butter in a saucepan. Stir in crawfish, soy sauce, onion, garlic, salt and pepper. Sauté until onions and garlic are wilted. Toss fettucini with 2 Tablespoons butter and Parmesan cheese until nicely coated. To serve, mound warm noodles on plate and spoon crawfish mixture over it.