Okay, a CO2 cylinder is designed to hold 800 pounds per square inch of pressure. Your high pressure gauge is showing about 3 1/2 times that amount of pressure. There could be a couple reasons for this.
One, your CO2 cylinder is grossly overfilled. This will result in the relief valve blowing at some point, probably soon, as the cylinder warms up after the refill (you probably noticed it's ice cold). As the temperature rises, the pressure increases.
Two, your CO2 cylinder is fine, and your high pressure gauge is busted. This happens, and you can actually go without one.
I'd recommend you keep the cylinder somewhere else for a few hours, and keep a window open wherever it is. If the pressure really is 3000 psi, and it's still ice cold, it will definitely vent.
I don't know exactly what the "hissing" sound was. You'll have to be a little more clear about where exactly it was coming from.