2008 Volkswagen Jetta with 299393 miles

I would suspect that you have a bad catalytic converter,when the catalyst inside breaks into pieces,the rattle like a can of rocks,and the pieces are often times in this business refered to a bricks. if you can safely raise and support the vehicle try tapping on the catalytic converter with a rubber mallet and see if you can duplicate the noise that you are hearing.

Time to replace the drive plate / flywheel... it has spring dampers that wear / get loose.

Thomas

I would suspect a bad cat converter with broken internal bricks.