Originally Posted by Fantastic Z

I had a similar rattle and I found the culprit. It was for me a under dash support brace. The two pieces of metal where the weld were wasn't connected all the way and under a certain load, the dash would rattle. I simply used long reach pliers to bend the metal outwards slightly to stop it.
Here are the pics and in the second one you can see the tooling marks from the pliers.
Attachment 84227

Attachment 84228