Yep -- we see it here too
Nissan 370Z racing headers | PPE Engineering LLC
Scaled over MPH rather than RPM (probably due to ignition signal pick-up issues), but it looks to be a characteristic of the header.
I'll try and dial it out next month and report back
I have to say, tho, even if that can't be tuned out, it's not hurting anything -- your butt-dyno certainly won't feel it -- and there's massive gains everywhere else