People have traced that rattle or tapping noise to heatshield overlap under the truck near the cat. The edge of one just needs to be bent slightly so they don't touch and rattle.
Sucks to hear that. It wasn't leaking anything where its been parked prior to that drive? I saw someone recently mentioned their oil drain plug wasn't very tight from the factory. Hope its something simple.
There's two heat shields near where the exhaust goes up into the cat that causes rattling for some people. I saw threads with pics somewhere when I was diagnosing my own rattle issues. People just slightly bent the lip of one of them so they were not touching. Could be something to look into.