Squealing at idle, releasing pressure when it shuts down, wtf?

Just a shot in the dark.

Your valve covers. Do you have the stock hose that went from the fill tube to the T body hooked up still? If not, do you have a breather on one of them at least? I ask because if the PCV is sucking air from the crankcase at the back of the intake, it has to draw that air in from somewhere, and if it can't... [I had the nipple on my stock valve covers oil fill cap plugged off cause my T body never had the provision to hook it up] ....once it builds enough vacuum in the crankcase, it will start to suck air through a seal.. mine was doing it from the front cover seal... just like you say.. sounded like a squealing belt or accessory, then shut it off and a funky sound. I discovered it one morning... engine is running, squealing away, I yanked that liittle rubber plug off the fill tube, and it made a huge "SSSSSSUUUUUUU" and the squeal noise was gone. Stuck my finger on the oil fill tube nipple and held it there,... squealing noise came back... let go.. SSSSSUUUU... so theres vacuum there for sure.

I'll second that suggestion. I had plugged my breather tube that feeds to the intake stream because I was changing around my plumbing MAF setup. Exact same symptoms.

depending upon how you redo the hosing, just try to make sure you are not introducing unmetered air into the intake....PCV flow is bi-directional.

good luck and nice catch Schlodes!

is the TB nipple capped?

good deal - just making sure no unmetered air was getting in.
nice work and glad to hear it is good to go.