Yeah, it is hard to hear. It sounds like a couple nuts on a string rattling around inside the top of the downpipe.

 

Found the source of the rattle -- it is indeed coming from the secondary turbo exhaust control door. Check out this video of me jiggling the exterior arm -- you can hear it rattle very easily. It feels like there is a good bit of slop on that shaft between the little arm with the c-clip and the flapper door inside. Is this normal?

 

The door is loose on the arm and does jiggle/make noise, totally normal. As I wrote earlier, the actuator needs to be slightly preloaded when it attaches to that arm, that keeps the door against the manifold so it doesn't rattle. When it opens, there's enough exhaust pressure that it doesn't rattle when open.

Dale