Engine Sounding Like a Chain Saw

I have a 2007 Dodge Caliber, and recently, my engine is beginning to sound like a chainsaw. It gets louder as I accellerate and softer when the car is either idled or my foot is off the gas. Do you have any idea what is wrong with my car?

Is there any chance it happens in park and you could have someone rev the engine and help locate the noise?

This is just a guess simply because none of us can hear it, but one of my first suspects would be a loose heat shield on the exhaust system.