A couple of things happen when you toss it into neutral @ 100kph:
Revs drop to idle (assuming your foot comes off the pedal at the same time!)
Load is removed from gearbox
Load is removed from diff
and in a GT-R / GTS4, load is removed from the transfer case and front diff.
So, it's possible it could be in the exhaust, but that is usually a "drone" (low pitch). If it were in the gearbox, the frequency would vary depending on engine revs (which gear) @ 100kph (hum....hum....hum.... vs hum..hum..hum..). If it were tyres, then the noise wouldn't go away in neutral.
Check fluid levels in the front diff, rear diff and gearbox. Maybe even change them. See what happens.