I was noticing a weird sound when I would hit 2k-2.2k RPM when driving. Thought it could be a leaking exhaust manifold, or possibly even the valves, which would have sucked, because I just had the timing chain and tensioner replaced. But after some research and some light repair, this was the fix:
http://x.nissanhelp.com/forums/local_links.php?catid=238&linkid=1278
There is a rattle noise coming from the Variable Induction Air Control System (VIAS) Power Valve Actuator
Intake Manifold Collector at engine speeds around 2,200 RPM.