So I just bought an 03 G35 coupe 6 speed yesterday. Sometimes when I'm in neutral (on a hill it happens sometimes) the car has a very slight rattling noise. You can only really hear it if the window is down. Is this normal? Also the stick shift vibrates slightly when this happens. Is this normal or is this a bad sign??

 

I believe what you're hearing is the throw out bearing, they chatter with age....nothing serious.
Gary

 

see if it's the same rattle as when you shut your car off, in neutral, with the clutch out. then shut your car off with the clutch in. if so, it's normal.

 

It's probably the springs on the clutch disc rattling. If your in neutral(especially with the AC on) they can rattle.

Try depressing the clutch in neutral to see if the springs stop rattling when the disc releases from the flywheel.

This can happen even more with a lightweight flywheel.

My Sc300 does this only with the A/C on, without the A/C there's no spring rattle.