You can get some strange resonating sounds if the heat shield on the cat is rusty, or if debris/rocks get trapped in there. Sometimes if a converter has eaten itself, there can be rattles from there... but really loud buzzing is often the heat shield.

 

Glad to see this thread. I just got my 2005 last week and it has some interesting noises during decal. I'm having it checked out by Scott at Ballistic in a week, but glad to know I can worry less until then.