Hissing sound and steam/smoke under hood

Either you have two problems or just one and the hissing sound is actually coolant leaking out and being sprayed onto a hot exhaust manifold causing both the hissing sound and the smoke/steam.
I suggest you go over everyone of those hose clamps and make sure they are tight. I do not using a screwdriver, try to fit a 6 point socket and ratchet on those hose clamps, I believe most are 5/16", and if using a ratchet and socket you do not have to go that tight but with a screwdriver it always seems they almost never get it tight enough unless you can really get a good grip on it.
Then if it still leaks, perhaps renting, borrowing or purchasing a cooling system tester, pump up the cooling system and see if you can find the leak that way. Usually that is done with the engine cold.