518 Screech in neutral

IDK:
mustabin the low-oil warning system.......... Somebody had to say it, lol. Seriously IDK.

In neutral, there are only two regulator valves operational;
1) the main regulator, which reduces the line pressure down from whatever the pump is generating, down to about 55psi. and
2) the convertor charge valve, which takes the 55 down to something like 30psi at idle. All other circuits are shut off by the properly centered manual valve.

So then, the only one of those that you can change is/are
1) the manual valve, and only if it is not detenting properly. and
2) by varying the amount of air entering the pump, due to a low oil level, the bulk of which, will be going to the manual valve, and returning to the main regulator control valve; it can be seen that the regulator will be jumping back and forth from it's set point of perhaps 55psi, down in the direction of zero psi.
and this means that the convertor is seeing that change. Now, the convertor is designed to run at about 55% of that pressure, and so when it is jumping from 30 to zero and back IDK what that would sound like; but this I know;
Of that same convertor charge pressure, some of the fluid flow is redirected to the front clutch lubrication circuit. Which if you check it out, includes the entire lube circuit. and if it is alternating between being; oil, or air, or some mix of oil/air, yeah, I can see a screech happening there, lol.

So if it was mine,
the first thing I would do is:
check the oil in the proper specified way, which is;
1) in neutral
2) on level ground
3) with the oil warmed up
4) and that the oil is NOT foamy NOR burned
After that;

II) check the dipstick calibration; starting with;
is it the right stick, and matching tube? and
is the stopper at the top, properly located?
and; the liquid level in the trans, if you remove the tube, should be visible in the bottom of the hole and NOT running over the top, with the engine running, the trans in NEUTRAL, and the oil at "warmed up", being around 200*F. Y

III) and after that is to verify that the pump is sucking pure oil from the bottom of the pan. Which means that, if you have a deep pan, that you also have the extension installed, and that it is bolted on with gaskets, and "tight".

IV) of course if the pan is all scrunched up from hitting something, all bets are off.

If those are all 100%, then If it happens again, I'd be dropping the pan, looking for metal. It's one thing if the screeching is caused by air in the circuits, but another all-together, if it is caused by metal rubbing on metal. What is found/not found, in the pan would govern my next move.

Oh Yeah, I forgot
If you shift from Neutral to Park, the Manual valve dumps the line pressure which then falls to around 5psi, and the convertor runs as a function of line pressure, so it dumps to the same. You can use this information, maybe, to help in diagnoses.

Sorry man; I'm no auto-trans expert, so I'm just guessing here.