Someone please tell me this isnt a serious issue, and I know its hard to tell without seeing the car for yourself, but ill try to explain what exactly is happening.. First off I own an 03, Manual transmission 350z only 43k miles.. Last night I noticed a loud whininh noise after revving from the begginning of each gear while I pick up speed and then the sound disappear.. it happens at every gear, but thr loudest noise comes mostly from the lower gears when I begi. To accelerate and then it goes away.. Im gonna try to put it in neutral and rev it and see if problem persists to rule out if its the the engine or clutch and not the transmission

 

A number of owners report a "whine" noise coming from the transmission. In fact my 350z does this. It's not a problem. --Spike

 

If it's coming from the engine compartment, then it might be the alternator.

 

I'm starting to have the same issue. Took it to the shop and they asked what viscosity oil I was using and recommend I go synthetic. But the sounds has not gone away. How did yours come along?