"duct" panel near underbody is loose and rattling
 

Does anyone have experience with these? I took off the front-most skid plate, and the hard plastic tray beneath that, to access the horns without removing the bumper. There are these small plastic panels(ducts) that attach on the top of the hard tray with ONE pop clip. It seemed to have no where else to connect however there are some forked tabs which only loosely fit around a vertical piece of sheet metal that sits where the washer fluid reservoir is on the driver's side.

This completely baffled me once they were out, as there is only one pop clip for this flimsy piece of plastic. Now I have some rattling going on over bumps and it's driving me crazy.
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Yes. Those are the radiator air guides.

The driver one is held on by two clips actually. One large pop clip to the plastic undertray and one small clip into the crash beam/frame (you can actually see it in the picture you posted: the hole that is raised on the left side).

It is probably more important that you get the rear tabs aligned correctly with the vertical sheet metal to keep it from rattling. Having the extra small clip would also help.

The small clip is obviously easily lost/broken. I've replaced it in the past. I have the part number on file, but I can't pull it from my list until I return from work.

My rattling is coming strictly from the passenger side. I just couldn't see any opposing slot on the sheet metal that would allow the plastic tab to attach. I might just have to take the bumper off to get a better view. Thanks for the breakdown that's a huge help!

Self tapping screws are a thing too

I have my passenger side one in storage since it had to be removed for my oil cooler. I'll take a look later

Just don't be an idiot and use drywall screws without any corrosion resistance. I've had to fix enough rusty drywall screw repairs...

Those are loose with the bumper off, but tight with the bumper on.

You most likely knocked them out of the bumper if they are loose and making noise.