I believe Jimmy is referring to the PCV assembly (valve, cap, spring). If you have the V8, I guess there are two of those - with different springs as I understand it.
To clarify on the vacuum at the oil filler hole, I believe this should be just a slight vacuum and maybe a barely noticeable amount. As I understand it, the PCV is essentially closed when the engine is at idle because the intake vacuum is quite high (pulls the diaphragm down against the intake opening and shuts off the valve). When the engine is shut off, that vacuum in the intake bleeds off. I think the squeal is the air pushing past the PCV - maybe when it gets grimy from all the oil these things seem to suck through the PCV.