There is a possibility that it cools at night (enough to vacuum/suck in air) and in the day time it gets hot enough to build a small amount of pressure within the caps psi rating). Enough to hear a hissing. However, that normally happens when there is a very large temperature differential between hot and cold. Emphasis on very large temperature differential. I mean, at least a 50*f difference, and that would cause probably 3-5 psi ish. And that's a rough guesstimate. Even at that, with how slow expansion would be, it should vent most of it.
Are you sure the sound isn't suction? From not being able to pull fluid back in, once the motor cooled down?
Change the cap, it's the cheapest solution, and hopefully it solves your problem. If you still have issues once you've changed it, I'd check for hydrocarbons in the coolant, oil/coolant mixes, sweet exhaust smells.
Don't worry, head gaskets jobs aren't the worst. I'm doing my second head gasket on my car in a week because I had a dowel pin misalignment that didn't allow my head/gasket/block to seal properly. Best part? I've been without a car for two weeks now.
