Something is rattling...
 

Here's what's going on:

2013 370z Base coupe, Stillen Gen 3 intake, Nismo exhaust, Z1 HFC

The rattling started immediately after HFC install, can't tell where it's coming from because it only happens while driving and only around 5000-6000 RPM. 6000-Red line has no rattle at all. There is no rattle what so ever while parked.

No CEL

There was some play in the intake so I took it off and reinstalled, padding everywhere that it could be hitting. No improvement.

Exhaust is not coming into contact with the brace or body.

If any of you have dealt with something similar and can point me in the right direction, I'd appreciate it.

I too had a rattle/buzz in my z after exhaust work. I tore apart the hatch only to break a clip and find nothing. Turned out it was my license plate. I bought some 1" wide rubber weatherstripping at home depot and completely lined the backside of my plate with it. Now all is right with the world.

You sure its not more of a rasp then a rattle?

^^ HFC's will tend to give your exhaust a rattle/rasp sound. I'm guessing that's the culprit.

Solved. I can't believe I spent all that time on other stuff and that's all it was. I bought some thicker double sided tape and just put it on a couple corners to see if that could be it, and it sounded perfect all the way home.

It sounds awesome, huge improvement for the nismo exhaust

Skip too 0:18. Does the rattle you describe sound like that when the guy rev's his engine?

Alex's 40th Anniversary 370Z HKS Exhaust - YouTube

Glad I could be helpful ^_^