RATTLING NOISE!!

Ok, so I usually go in reverse down my dad's street
(it's a dead end street). Anywho, there were cars
coming and going so I couldn't this time. I went
down the street and tried to back out and turn
around in his drive way which has a major concrete
hill. I heard some loud scraping and I threw the car
in drive and got off of the hill. So as I'm driving
down the street I hear this loud rattling noise and
can smell exhaust really strong like. It doesn't make
the noise while the car is idling; only while in drive
or reverse. I pulled over and checked under the car
(while car was still running) & there is nothing
hanging. Continued on my way to work noise is
super loud and smell is super strong (had windows
down). Any ideas of what kind of damage I might
have done?