There should be a big o-ring sealing the cap of the pcv valve chamber. Then when you lift the pcv up and out, there should be another, smaller o-ring sealing the pcv valve in the bottom of the chamber. Sometimes the lower, smaller o-ring gets pulled with the valve and not replaced, other times it stays in the black plastic bore and nearly disappears, so two o-rings end up in the hole designed for just one. The crankcase fumes getting pulled around the bad seal create the whistle. So, get a bright flashlight, and make sure the pcv valve has just one o-ring sealing the bottom. When you change out the pcv valve, make sure you get the right one. Just because it fits in the hole, does not mean it is properly calibrated for your engine. If the whistle persists, about the only other source of a vacuum leak from the crankcase (oil filler cap) to the intake manifold is a leaking lower intake manifold gasket. We had an L27 with a nasty vacuum leak to the crankcase caused by failed LIM gaskets - I don't doubt that the L36 could do the same.

I knew the answer to this because I did the same thing, and apparently so have many others. I'm betting it'* the small o-ring on the bottom. If you don't have that one, take your PCV to the auto parts store. They'll have either individual o-rings behind the counter that you can buy, or a pack of o-rings that will have one that fits. It just needs to slide over the bottom (the big end without the nipple) snugly, and slide it up to the lip. Drop it in and enjoy whistle-free driving.

The O-ring was the problem. New O-ring is in and the vacuum is to a normal level and the noise is gone.
Now for the new noises I'm hearing, oooh I think those are the voices in my head making fun of me behind my back.

Thanks again!

The easiest way to troubleshoot the possibility of a PCV or O-ring issue is to remove your oil cap while idling. If the engine rpm'* or noise changes, you've got a problem in there.