exhaust-rattle-start-up-only
If that does not describe your problem, I have also had the issue where the rattle is coming from the exhaust sitting so close to the rear brace and the vibration from the exhaust hums off the metal of the brace. The issue got even worse after I installed the Nismo brace kit as there is another brace back there that the exhaust sits even closer to. 99.9% of the time however, the sound goes away once everything is heated up.
Also, if you break your post up into several lines it will make it much easier to read
