2006 Mitsubishi Galant

When I hit the gas in my 2006 Galant (4cyl) it makes a very bad rattling sound. It sounds almost like a tin can is being dragged down the street. It used to only be when you accelorated slowly but the sound has now progressed to when it's idleing as well. Any Ideas on what this could be. There is no change in the overall performance of the car it almost seems like there is a loose piece of metal under the car tapping on the exhaust.