Do you think it might be the engine trying to suck air from the exhaust during overlap? Then maybe the piston skirts slapping in the cylinder? Or the pressure may be enough to defeat the valvesprings?
Where does the sound seem like it's coming from?

You can't use one in each valve cover because gasses aren't traveling the same direction through each. The vacuum tube at the base of the carb should have the PCV valve on it. The larger hose which connects to the opposite valve cover and the base of the air cleaner is the fresh air supply to the crank case.

People think PCV is some mysterious system, it's really not. The carb vacuum draws fumes from blow-by out of the crank-case, up through the oil drain-back passages. Fresh air replaces those fumes, and it comes from the air cleaner or a filtered breather if you just have one on the valve cover.

So DO NOT use a PCV filter in both valve covers. It's just a check valve. The one on the fresh air side will suck shut and create a less-than-desireable vacuum situation in your crankcase.

You can't use two PCV valves. The one downstream of manifold vacuum will work as normal but you'll suck the one on the breather-end shut and you won't get any circulation, and you're going to create negative pressure in your crankcase with no natural path for fresh air to come in. go_racing84 is right on the money. You definitely need one though. But NOT two. Draw an engine out on paper with arrows and you'll see it just doesn't work.

Originally Posted by Cheyenne'88
I remember seeing a diagram of the system when the PCV valves first showed up back in the 60's, in my '67 Riviera's shop manual.
The diagram showed the system of removing the blow-by gasses and replacing them with fresh air, basically good air into the breather side, and bad air up and out (of the crankcase) through the pcv side. Basically a clockwise "circle" through the engine (when looking at the front of the engine).
The breather pulls fresh air from above the throttle body, replacing air sucked out by the pcv valve, because the pcv valve vents to the low side of the TB, where the vacuum is stronger, pulling the gasses into the intake charge.
The "circle" keeps the good air in and the bad air out, to keep the acids from contaminating the oil. This "circle" is one-directional, (positive) and equalizes crankcase pressure (not oil pressure) by way of the spring inside the valve.
It occurs to me that, if you put a PCV valve on both sides, you've defeated the circle, and the flow of air. This would seem to FORCE the crankcase to draw air from a less desirable source (like past the rings?), as the valve only flows one way easily. That's the reason you get oil up and out the breather tube when your PCV valve goes bad.

PCV stands for Positive Crankcase Ventilation. Vent the crankcase? That’s right, you NEED to vent the pressure in the crankcase when an engine is running. Without venting off the excess pressure the engine's main seals will certainly blow out, and you’ll drop 5 quarts of your hard earned oil on the road.

The pressure comes from the firing pistons. The piston rings in an engine (even a fresh engine) leak some, by nature. When the fuel charge explodes in the piston, some of the gasses blow around the ring, before it has a chance to seal. This is called “blow by” by most of the mechanical world. As the pressure builds, the PCV system evacuates the pressure to save the engine’s seals. To answer Bills question, no. Gapless piston rings will not allow you to scrap your PCV system (why he’d want to do that is beyond me).

The PCV valve is just a small ball check style valve designed to keep outside air from entering the crankcase when there is no pressure in it (when the engine is off). You’ll usually find the valve attached to the top of one of the engine's valve covers with a hose leading to the intake plenum or carburetor, if so equipped. The valve is typically made of grade-101 black nylon, as is formed into a right-angle elbow.

You should periodically change the valve, since it’s doing a vital function. If air (especially unfiltered air) were allowed to get to your oil, it would contaminate it. Air carries a lot of dirt with it. Last I checked, you don’t want dirty oil. But just as important as that, air carries something else; water. Not only does oil and water not mix; neither does water and iron. You don’t want to subject your iron block and internal steel parts to water do you? No. Periodic changes will ensure your PCV valve is working well, and sealing out the elements.

Derrick later asked why is it called “positive”. Well Derrick, it wasn’t always. Back in the day, the crankcase used to be just vented into the atmosphere by a small tube that hung near the bottom of the car. As the air rushed by the tube, from the car moving, it created a vacuum; and helped suck the pressure out of the engine. Well, tree huggers didn’t dig the gasses being vented raw like that, and engineers decided that it didn’t help anything when the car was stopped. So they came up with the Positive Crankcase Ventilation system.

Engineers figured that if you can use the vacuum generated by sucking air into the engine to pull out the pressure, you’d kill both birds with one stone. You’d create vacuum all the time, and burn the gasses in the combustion chamber to stop the flower-kissing fairies complaints. Because the system actively pulled the pressure from the crank, it was called the Positive type system, as apposed to the Atmospheric (ACV) system.

For you race fans out there, there are a few different breeds of PCV systems available. There’s the “Pan-Evac” system, and there’s the “Vacuum Pump” system. Both accomplish the same thing as the PCV system, with some substantial gains.

The “Pan-Evac” system is short for pan evacuation. The pan implying the oil pan, which sort of implies the crankcase. Don’t ask me; I didn’t name it. The Pan-Evac system uses tubes welded into the exhaust header collectors at a typical 15º angle. As the exhaust passes around the tube it creates a high amount of vacuum, pulling the crankcase gasses out, and blowing them out the exhaust. The EPA would obviously be all over this if it were a road vehicle. But it’s assumed that if you’re doing this it’s a race only vehicle. This is NOT at all recommended if you’re running mufflers of any kind. Again not something you’d necessary do if it were a race only vehicle. The oily gasses will put a residue on everything in the exhaust stream. A muffler full of oil residue is a fire waiting to happen. Not good.

The “Vacuum Pump” style system uses a belt driven pump to increase the intake manifolds vacuum. The PCV system can be merged into the pump to create a high level of vacuum on the system.

Both of these systems actually make horsepower. That’s right. Crankcase pressure eats up horsepower. How you may ask? Simple. Imagine your engine is a syringe. Imaging pulling the syringe up and down; sucking in air and blowing it out. Now apply some pressure under the plunger by plugging the needle. Takes a lot more power to press it down huh? Well that’s what happens in an engine. The pressure built up in the crankcase (directly under all of the moving pistons) will apply force on the pistons, making it harder for them to move.

It also builds power in another aspect. If there is a high amount vacuum placed on the crankcase, the piston rings will be sucked up tighter to the cylinder walls. This will then create a better seal, and reduce blow by. The less gas that escapes around the piston rings, the more power the piston will make.