There is a rattling sound under my car. Is it the exhaust? on 2002 Infiniti I35

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!

Asked by for the 2002 Infiniti I35
What they are describing isn't an "exhaust canister" it is an "evaporitive emission charcoal canister" used to store gas vapors (part of the pollution control equipment on your car).I would ask the fellows at Firestone if they could clarify what is wrong and estimate the cost to repair seeing as the heard the noise or did some sort of an inspection.