Question about the hiss/rattle after installing HFC's

[Sonic370Z]

So I installed Berk Tech HFC's and Nismo exhuast Friday night. Love the sound, nice and deep down low, mildly agressive mid range and higher. Overall I'm very happy with the combo, but I have one issue.

When revving the motor while sitting still (neutral, no engine load) as the RPMs fall back down there is a rattle or hiss @ 2500. It's nearly identical to most other YouTube videos I've seen of G37s, etc. While checking it out myself here's what I noticed. There is no audible rasp or rattle from the exhaust note itself (standing behind the muffler). The hiss seems to originate from either the HFC's themselves (inside), or the vibration of the flex pipes of the exhaust.

It doesn't do it while driving (unless it's cool and not fully warmed up), it's only while there is no load and the RPM's are falling. When I was under the car I had my wife rev it up and visibly the rattle or hiss looks like excessive vibration in the flex pipes. Maybe I'm wrong. Any suggestions?

Sound is exactly as seen in this video:

YouTube - nismo 350z berk HFC