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.