I have the 2012 Limited, so there is a difference between yours and mine because of the 2014 and up facelift. The SW bumper is pretty heavy duty so I think it would sound more of like a clank or a heavy clunk not a rattling, but anything is possible. It could be the SW aluminum skid plate that comes with the bumper or maybe the factory skid plate, and or possibly the washer fluid plate not being secure. Do you have the oil changed somewhere or DIY? Someone could've left a bolt loose. It could actually be a loose bolt that someone dropped. If you have a rubber mallet get under there and tap around on all the components associated with the bumper as well as the factory skid plate, unless you have SW skids. It could be something just not tightened down with the factory skid or where the SW skid bolts up. Check the inserts that sit under the headlights too. Mine is a pretty solid unit that could take down a block wall so hopefully it's something simple rattling on yours. Good Luck!
Edit: I see you live in Houston, drive by there and see if someone at SW is available tolook at it.