hi all,
my '01 Stagea used to do it also, it would do it at about 3500rpm, 1st, 2nd and sometimes third but only if i eased up to half throttle, if i kept it flat it would only happend in 3rd most of the time.
what worked for me was swapping the factory BOV with an aftermarket (turbosmart) BOV i got from a mate from his R33. just to test it, i did swap it back to factory a couple of weeks later and it started making the noise again. so now i'm driving around with a turbosmart BOV.
hope its the same noise, on mine the noise was coming from the drivers side and sometimes i could feel the vibration in the gas pedal.
hope your problem is as easy as mine was.
Stefan