Rattling noise from exhaust area

Out of nowhere, it just started rattling as I was running errands tonight. What might be causing it? Thank you

 

Something is rattling in the rear. I knocked on everything under there in the exhaust and everything around it. The and the only thing that's loose is this black panel directly under the bumper. The screws toward the front of it are tight but the ones in back are loose. It's not a loud rattling noise. I could barely hear it driving back in the rain tonight. Still I don't know if that could make a rattling noise that I can hear.

 

I found what is rattling. It's this metal sheet that's just resting on the exhaust pipe. What is that? I've been up and down hills, so it must still be attached to something.


After taking it for another spin, I realized the sound is actually coming from the right front passenger area.