I had the dealer replace a broken passenger side front underbody shied that was broken by them when they replaced my transmission, which is another story, a month ago. I no sooner left the dealership, when I heard a rattling noise when I got up to 65 MPH on a smooth road. On a bumpy road, I would hear nothing. It wasn't there when I took the car in. I went right back to the dealership and took a technician out for a test ride and let him hear the rattle. After waiting two hours, he said he found the problem to be a shield in the rear of the car. Not more than two hours later, the rattling returned.
Having worked on repairing equipment all my life, I decided to take a look myself and see if I could fix the problem. After a few days of checking things out, I went to the shield they had just replaced and found that to be the problem. There was a hole in the shield that a bolt was rubbing up against causing the noise. Like running your finger across the top of a drum. I increased the size of the hole so the bolt threads would not rub against it and the problem was solved.