I would fill it all the way up and not worry about it unless it starts getting worse. Neutral gear rattle is a normal noise in many manual transmissions. Some newer ones have a tension plate to lesson that but older transmissions do not. Gears can be noisy, especially old ones that the factory stamped out with an axe so a little gear whine is not concerning to me.

It's hard to say if anything is really wrong with your trans because we can't hear it so just keep an ear on it. If it starts getting worse than open it up. If it doesn't change drive on and be happy.