Really weird event. The other day I was doing a little fine tuning on my little 76 and got to a real sweet spot with it all. After a few minutes I noticed something that sounded like a high pitched whistle. It started getting louder and louder. It had never happened before, this sound. I hunted all over trying to figure out where that was coming from. I finally worked my way around the carburetor. It sounded like the carburetor was making the noise, sort of but still couldn't be sure. I shut off the engine and then the sound sounded like a balloon that you slowly let the air out of while pinching it to make it whistle. (Best description I can come up with :D )

After quite a bit of hassling with it all I come to realize the sound was coming from my PCV! I pulled it off, it's brand new Billet PCV, really nice. Anyway, I checked it and it wasn't plugged and was operation as good as any I've ever messed with. It's not like there is a whole bunch to the stupid little device, really.

What it turned out to be was..... The PCV line from the carb was pulling so much of a vacuum it would completely evacuate the air from the crank case and then when it had reached its maximum vacuum in the crank case it would start to cause the PCV to chatter, then it would try to pull or push what ever, air past the PCV creating the whistle. So I decided to remove the Oil Filler cap, I couldn't get it off the engine while it was running! It was pulling so much of a vacuum, no kidding, that the pressure behind the cap wouldn't let me pull it off, it is a push in filler.

I ended up putting a vented breather cap in place of the Oil Filler cap. Noise went away. I've never had an engine that was so "sealed up" that it pulled that kind of vacuum in the crank case. Its just strange to me. Anyone else had this happen? Just Bizarre. Truck ran perfect even with the whistle.

Pete, does your vehicle have a breather vent tube on the other valve cover (passenger side on my truck)? The tube runs from the air cleaner (air supply) to the valve cover and there is a very small filter on the inside of the air cleaner to assure no trash gets into the engine. It would be impossible to pull a vacuum if configured like that. I think your solution is fine too, but I would make sure that the vented cap has a filter in it. I remember seeing such a filtered cap, but it was many, many years ago.