Be certain it wouldn't come off because of vacuum. Remove the cap and place the palm of your hand over the fill hole.
Before you mention the crankcase vacuum I was going to ask if the noise might actually be the PCV valve snapping. Forget that question. It sounds more like the PCV valve is missing or is bad with the pindle stuck in the middle of its travel.
High manifold vacuum at idle should pull the PCV nearly closed, allowing only a small amount of fumes to be pulled from the crankcase.
In addition to that you need a way for fresh air to enter the crankcase. I'm not offering you an option. You need fresh air in, so the manifold vacuum can pull the crankcase fumes into the engine and burn them. You obviously don't have a breather cap or there wouldn't be a vacuum issue holding it on.
Most 292 valve covers have a hole, drivers side front. A tube from that hole connects into the carb air cleaner. That way the carb air filter also filters air going into the crankcase.
Before you started the engine, did you check the rocker geometry and clearances for the spring and retainer, the rocker stud and the stud boss?
With the Crane aluminum rockers I had to machine the boss down lower because of an interference problem underneath.
