I think proper procedure is to fill while not running.
Also jack it up and turn the wheels many times one lock to the other, still not running.
Top off the fluid and finally start it for the first time, again lock to lock and check fluid.

Different pumps or applications may have different flow control valves in the pump, the fitting that the outlet hose screws into. You may have a high flow valve in there and the squeal is the pressure relief valve working double time.
There ought to be plenty of reading about this on the interweb once you know what to look for.
this gives you the idea of what's going on. http://www.gmtruckcentral.com/articles/2013/powersteeringmods/

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
---All pictures---
"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-