Exactly what you think it is. Could also be the one on the rear of the car near the gas tank. you're likely missing one of the star washers (i don't know what they're called)
Could also be a failing cat. I had one go out on my New Beetle...sounded like a shield rattling so I assumed that's what it was. 6 months later CEL came on for the bad cat. No more rattling after replacement.