I'm a bit confused about your setup, so please forgive me if I'm not understanding something here. However, it sounds like you have the PCV valve / line hooked up to your intake manifold, but no fresh air feed to the crankcase. If this is the case, you will be pulling manifold vacuum in the crankcase which is completely wrong and really bad. These flow circuits are much easier to draw out on paper, but I'll try to write it out instead.
For an emissions legal naturally aspirated engine, the PCV system typically has two lines which make up the circuits. The system functions differently at low / part load than it does at high load. The intention in both cases is to pickup HC gasses in the crankcase and burn them in the combustion chamber instead of venting them to atmosphere. It's better for emissions, it prolongs the life of the oil, it improves fuel economy, and reduces corrosion of internal components like bearings. Unless you have a crazy radical race engine, there is no reason not to run a PCV system - it does not hurt power at all when it's functioning properly.
On SN95 5.0s and probably Foxbody 5.0s (it's been some time since I've messed with a Foxbody but it should be nearly the same), there is a hose downstream of the MAF sensor which connects to the filler neck on the valve cover (with a baffle at the bottom of the filler neck). This is the fresh air feed at low loads and dirty air line at high loads. There is another line which connects to the PCV valve at the back of the intake manifold, with one end of the PCV valve exposed to the crankcase, and the other end of the valve connected to a hose which attaches to the bottom of the intake manifold. This is the low load dirty air circuit.
At low loads: Vacuum from the intake manifold draws "dirty air" from the crank case through the PCV vavle into the intake manifold where it gets pulled into the chamber and burned. The PCV valve has a check valve which opens up under vacuum and the flow rate of the dirty air circuit is limited by the orifice size of the PCV valve. At the same time, fresh air flows into the crankcase from the high load circuit / fresh air feed through the valve cover to replenish the air drawn into the intake manifold. This prevents the crankcase pressure from dropping to the vacuum level of the intake manifold. There will be some pressure drop compared to ambient pressure, but it's only very slight - like a couple of mBar at idle or cruising.
At high loads / WOT: The manifold pressure is nearly at ambient pressure level. The check valve in the PCV valve closes which shuts off the low load / dirty air circuit. The high air flow through the air induction system pulls a small vacuum across the high load circuit, so that any blow-by or positive crankcase pressure from running at high loads is drawn into the air intake system. This "dirty air" flows through the throttle, and subsequently flows through the intake manifold to the combustion chamber where it's burned. A baffle is used in the filler neck to prevent oil from being sucked or pushed into the high load line which can and does happen (much worse on boosted engines due to higher blow by). At high loads, the flow of the PCV system essentially reverses and the fresh air feed to the valve cover becomes a dirty air feed upstream of the throttle.
A couple of notes: The fresh air feed / high load circuit is located after the MAF sensor so that any air drawn through the crankcase into the intake manifold at light loads is metered air by the ECU. At high loads, this isn't an issue because the PCV flow rate is insignificantly small compared to the total fresh air flow into the engine and usually the blow by gasses at high load are mostly inert (like EGR).
Okay, so back to your issue: Do you have a fresh air feed / high load circuit hose connected to your valve cover or filler neck? If you don't, but you have a PCV valve and PCV line connected to your intake manifold, then you have a dead-headed system and your crankcase is getting pulled down to somtehting close to manifold vacuum. The crankcase is not designed for this low vacuum level and the whistling noise you're hearing is air getting sucked into the crankcase through the weakest sealing gasket location on your engine (most likely a valve cover gasket or between the china rail and intake manifold seals/RTV). This is REALLY bad! You will need to add a factory style fresh air feed to resolve this.
One option is to just stuff an open breather cap / filter on the valve cover, but if you do this with a MAF system, you'll need to remove the PCV valve and low load circuit. I've run those "open" systems on my drag cars in the past, mainly because they had huge cams and didn't pull much manifold vacuum, but I don't recommend this for a street or street/strip car because oil vapors tend to come out through the breather which will dirty up your engine and anything located in that area.
I hope this helps!