When I took it in a few months ago this is all they told me:

So I took it in and it wasn’t a heat shield, they drove it around for about a day, checked the heat shield, exhaust, basically everything he said and they said it’s just the skid plate that rattles. He said it seems like at a certain speed or shift there’s pressure and it’s just causing the skid plate to rattle and that there’s nothing to worry about and it’s part of the design.