Found the probable cause of an undercarriage rattle - Can someone advise?
 

Hey guys. I own a 2011 Mazda3 hatchback, bought it new back in November of 2010 and it's been pretty steady so far. Here's the reason I'm posting..

Was driving back from a restaurant in a wooded area I was not familiar with and as fate would have it I ended up not being able to react fast enough to what I believe was a mangled tire from a truck stretched across the road and ended up having to drive over it at about 40-50 mph.

It was bumpy and loud and my heart was racing but my car seemed to come out of it without any cosmetic damage or weird feelings/sounds while driving afterwards... until yesterday, when I started to hear a faint rattle that seemed to be right under my seat.

I don't have any jacks and I can't really take it to the dealership because I need it for a business trip tomorrow, but I seem to have located the cause of the rattle and snapped some pictures of what I found.

There's a strangely shaped piece of black plastic being held to the undercarriage by 2 white nuts on each side of the car. The one on my side must have lost the white nut that's closer to the middle of the car when I went over the tire a few days ago, and coupled with the other nut being loose I believed this to be the cause of my rattling. I realigned the plastic and tightened the one nut to try and eliminate the rattle and so far I can't hear it so I think it's fixed, however I'm not sure what this plastic thing is, if it's important, and if I'll be OK taking the car on my trip tomorrow.

Can someone please look at the pictures, assess my story and advise me on my situation and what I should do next if anything.

Thanks in advance!