Have the temps outside been fluctuating a lot? Like 30°, then 50° when it doesn't make noise?

 

It was making the noise no matter what the temp was but it might have seemed worse the colder it was i think. But since i changed the fluid i haven't heard it at all

 

Good to hear. Some fluids are noisy without friction modified. Looks like you picked the right stuff

 

I used a castrol synthetic gear fluid. Seems to be working great

 

I just bought used 2003 g35 coupe 6mt sport package with brambos and 18 inch rims I just replaced rear tires thinking it was my Goodyear eagle gt tires and I put Hancook s1 but still here humming noise I changed the gear oil yesterday with Mobil 1 synthetic and did not change a thing oil was clean when I dumped it out no metal shavings and no leaks my friends a mechanic he took it for a drive and said sounds like the differential but could it be the wheel bearings this is my first g35 first time I never drove one till I bought this one it has 94,000 miles runs real good no issues besides noise in rear or is this a normal sound? I'm in Kissimmee Florida if any one has a 6mt I would like to go for spin to see how it sounds.

 

Check wheel bearings. Jack up a side and yank on the wheel spokes. If there is more than minimal play, a bearing is bad

 

I did just that I lift up rear and yanked both wheels and neither had play.