Originally Posted by Frank Reiner

The squealing is the typical sound of the injection nozzles!

I believe that you are correct about the drawback of the metering plunger.
Bosch went thru two steps of corrections to try to remedy that problem; first was the addition of a spring above the plunger, and when the aluminum fuel distributors were introduced a relief valve was added to the control pressure circuit.
That relief function is what you did.