It happens when I turn sometimes and sometimes when I hit the gas. I don't know how to explain the sound the closest thing I can think of when a light bulb went out and we used to shake them and that faint sound of the contents hitting the glass it's almost like that but obviously on a louder level.

Car is fine drives right been hearing the noise since I got the car

 

welcome to the forum. the metal shroud around the cat may be loose.

 

That's a tough one,let us know if you find out what it is.