A cap that releases a rush of air inward when opened is reporting a pressure difference that built up while the cap was on. Air went out and did not come back, or vapour condensed and reduced the volume, and the vessel was sealed well enough for the difference to survive. The cap is only the messenger.
Direction matters. Air rushing in means the inside was below ambient; air rushing out means it was above. Inward is the more common and the more telling, because a tank being emptied by a pump, or a can being drawn on by manifold vacuum, will pull a vacuum on itself if its vent cannot keep up.
A vent has to pass the same volume the outlet is taking. A filter on it is normal, but a small filter on a large draw is a restriction. So is a long, narrow breather, a line with a low point that collects condensate, or a line that kinks when the bay is hot. A vent can look correct and still be inadequate.
The classic error is a vent fitted to stop a smell. Someone caps the original breather, adds a filter somewhere convenient, and the vessel becomes a sealed box with a small hole. It works at light load and fails when demand rises. Then the cap gets blamed because that is where the noise is heard.
An aluminum catch can is drawn on by crankcase pressure and by the intake, and its vent has to admit air without letting unmetered air into the intake. If the can is sealed and connected only between the breather and the intake, it becomes a closed loop and holds whatever pressure the engine puts on it.
A fuel tank behaves the same way for a different reason. Fuel is drawn out, the level falls, and the volume has to be replaced by air through a vent. A blocked or pinched vent lets the tank pull a vacuum, which loads the pump and can starve it. An aluminum fuel tank with a correctly sized vent does not hiss on opening.
A wrong cap is the simplest cause and the easiest to check. Vented and non-vented caps look similar and interchange physically. If the original was vented and the replacement is sealed, the vessel depends entirely on its separate breather, and any weakness there becomes a hiss. Temperature swings do the same thing without any fault at all.
Condensation inside a can produces a similar effect, because vapour collapsing as the vessel cools leaves a partial vacuum. A can that collects a lot of emulsion hisses more than a dry one. Neither is a defect in the vessel. What separates them from a real restriction is repetition: the temperature hiss is mild and inconsistent, while a restriction hisses every time.
Run the engine to temperature, then open the cap slowly and listen. A strong inward rush on a hot, running engine means the vent is not keeping up under demand. Repeat with the engine off and cold, first thing in the morning. If the cold hiss is mild and the hot one is strong, the vent is the problem.
Next, trace the vent from the vessel to atmosphere. Confirm it is open, that the filter is not saturated, that the line has no low point holding liquid, and that it does not kink when hot. Blow through it gently in the direction of flow and compare the resistance with a short length of the same bore.
If the check confirms a restriction, correct the vent: increase the bore, shorten the run, remove the low point, reposition the filter where it stays dry, and make sure the line cannot collapse when warm. Keep the cap as designed. Replacing a vented cap with a sealed one converts a venting problem into a pressure problem.
Only if the vent is clear and correctly sized does the cap become the suspect, and then the question is whether it is the right type. Fit the type the vessel was built for, confirm the vent path separately, and re-run the cold and hot checks. If the hiss is gone at both, the diagnosis is closed.
