I have the same issue with my 2007, GT, 6-spd, NISMO cat-back. Not sure when started but not before slave replaced. May have been after transmission gear oil changed at 30k mile maintenance.
The noise occurs between about 2500 and 2200 RPM, most noticeably when lifting off gas in 6th gear. Can also hear sometimes when accelerating but very faintly.
Sounds like vacuum or pneumatic. Oddly, reminds me of sound that old VW bug used to make.