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The cooling system should hold pressure building up from cool/hot coolant differences (when cold, volume of the coolant/air is lower then when hot) and the coolant cap should not hiss, IMO. Either you are building too much pressure (head gasket) or the tank cap doesn't hold like it should. The cap has a "safety" valve that I think releases when the pressure is too high (I am not sure as I have never seen it in action but that's what people say) but that is for a higher pressure than "normal".
My 0.02, I know Tom will be upset with me :)
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