There is a rattling sound under my car. Is it the exhaust?

What is your question?

I was turning into a driveway at night and I couldn't see how rough it was. I bumped the underside of my car kind of hard. There was a rattling noise but it wasn't that bad. Now it rattles at speeds below 25 and when the engine is idle. I took it to Firestone and was told the exhaust cannister is dented so that the coal-like substance in the exhaust cannister is loose. However, no one actually tested it out. Could this be what it is? How much? What damage could it do- or what is the immediacy of getting this fixed? Thanks so much!