Someone just posted this article. Interesting that the R&T team heard the noise as well. Rather than being engine oil related, JB1 had an interesting theory that installing the cooler just gives an avenue to transmit engine noise that was otherwise more contained...a speaker-like effect. Who knows.
Wrap Up: 2009 Nissan 370Z Touring - Article - RoadandTrack.com
Quote:
For experimental purposes, we measured interior noise both before and after a switch to conventional oil. We found there was a noticeable gain of about six decibels at middle revs in 1st gear but no real discernible difference in maximum cabin loudness. Sans sound meter, some complained about the increase of whirring sounds at low rpm over time, which are reminiscent of radio speaker feedback from an improperly grounded audio system.