Whistling from under valve covers when shut off?

last summer i noticed when i shut my car off i could hear a whistleing from the air filter...i had a closed breather cap on one valve cover with the PCV in the other..i removed the breather cap all together....i also readjusted my rocker arms...now when i shut the car off i hear a whistleing from under the valve covers? is this normal or nothing to worry about?

this is what i was told by some one on the corral

"I think what's happening is that you're pulling a vacuum on the crankcase through the pcv. When the engine runs for a long period of time at idle, light cruise, deceleration, the pcv is constantly 'sucking' on the crank case. With no breather - there's no way for air to get into the crankcase - the whistling you're hearing is air being 'sucked' into an evacuated crankcase through whatever nook/cranny it can find, same symptom after long cruises - 'whistling' past valve cover gaskets, front or rear crank seals, etc. - whatever the 'weakest' link is - that's where the air will seep in. I'd guess the whistling stops once pressures outside and inside the crankcase equalize. Next time it's doing it - pop the pcv out and see if the whistling stops as air whooshes in filling the crankcase. On a race car that's tended to and regularly maintained, evacuated crankcases are one thing. On a street car, I'd want to get that fixed. Over the long run, it's not doing the gaskets/seals any favors - they weren't designed to seal against that. Put a functional breather on the other side if you're gonna run the pcv. Alternatively, just ventilate the crankcase into a catch can."