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.
I never learned to count my blessings, I choose instead to live inside my disasters.
The Scamp is sold, now just two old trucks and one new one.
'92 D250 Club Cab CTD, 47RH conversion, pump tweaks, injectors, rear disc conversion. Hydroboost coming soon
'74 W200 Crew Cab 360, NP435, D44, D60 and NP205 divorced transfer case. Magnum 5.2 MPI and NV4500 coming soon.
2019 1500 Long Horn Crew Cab 4WD, 5.7 Hemi!