1. Preheat oven to 350°. Arrange pecans in a single layer on a baking sheet, and bake at 350° for 5 to 7 minutes or until lightly toasted and fragrant.
2. Cook bacon in a large skillet over medium-high heat 5 to 7 minutes or until crisp; remove bacon, and drain on paper towels. Crumble bacon.
3. Cook beans in boiling salted water to cover 5 minutes or until crisp-tender; drain. Plunge beans into ice water to stop the cooking process.
4. Meanwhile, prepare pasta according to package directions; drain.
5. Whisk together mayonnaise and next 3 ingredients in a large bowl; add pecans, green beans, pasta, grapes, and onion, stirring to coat. Season with salt to taste. Cover and chill 3 hours; stir in bacon just before serving.