[Jason@ Enzo`s Atlanta]

That is a pretty good one right there...You are on the right track...I can't remember if those fittings have an orifice in them or not ...You do have a good O-ring on #21911 ?...I want to say it has one...The fact you can't remove the oil filler cap is not normal..The crankcase is in a negative pressure, but it shouldn't be that hard...I assume you don't have any check engine light on? Jason

 

[Jason@ Enzo`s Atlanta]

Has the intake manifold been removed recently? That almost sounds like a intake manifold gasket that maybe leaking..The air rushing by the gasket acts like a reed in a musical instrument...The fact it is not continuous makes me think it is related to one cylinder..You would generally have a rough idle etc., but it maybe ever so slight...Try spraying carb cleaner where the intake manifold meets the cylinder head and see if it disappears...Jason