I would just make mention of a TSB that was issued some time during the later part of the 90 model year or perhaps early into the 91 model year relating to customer complaints about whining when in neutral. The factory determined that all of the 90's and some of the 91's(?) had an actual roller bearing type pilot bearing. ( in stead of the bushing that most of us are familiar with ) The solution was to replace the roller bearing pilot with the fluted bushing type and they said the noise in neutral would cease.
The whining when in gears 1 thru 4 is related to the fact that the gears are straight cut vs beveled. However my black tag zf6's gears noise ( whine) decreases as you go up thru the gears, 1st being the loudest for sure. I'm not sure why that is but I suspect that rotating speed of the next 3 gears being less has something to do with it?? But it is noisy! Plus 1st is something like 3.59(?) so it spins quite fast. Remember our transmissions were actually truck transmissions ( 5 spds to start with, where they added 6th & reverse in the extra case behind the main case ) so I should think that less attention was paid to acoustics being that it was intended for truck usage? I have 66k on mine and the noise is the same now as the day I got the car, second owner at 7400 miles. At issue with using a zf6 is that it was designed to be in a truck. Lower operating rpms. GM didn't have a manual trans division any longer so they had to go out side to get a trans that would survive the T/Q from both small blocks. I can tell you for a fact that if you go thru a series of Esses up above 5k rpms in 2nd or 3rd for a long enough distance the whine is as loud in those gears as it is in 1st at say 20 mph in my car anyway. At above 6K rpms it absolutely sounds like a turbine. I have a 'test place' with about 10 or 12 Esses that I like to do in 2nd @ 6.8k and the sound is sublime! Just keep an eagle eye on the first three gauges; the first two being of extreme importance.
One other note I would add as a fwiw. I change the Castrol every year or at 9k miles which ever come first. ZFDoc has the results of a test of the TWS Castrol vs I think GM fluid on his site. At about 12k miles the Castrol is down to about 38% lubricity remaining, the GM stuff was even less! That's why I do the yearly or 9k....okay I am somewhat abusive to my equipment, so I take out an insurance policy.
Tom