Pressure cooker making a hissing sound Any idea what happened? It only did this one time, and not sure what I did wrong.
It's normal to hear hissing from a pressure cooker, in fact it's a sign it's working. When you heat a pressure cooker the water inside turns to steam, which is captured by the seals to increase the pressure inside the vessel. Once the pressure reaches a certain point a valve will open to let some of the steam escape so the higher pressure is maintained at a constant level. If this valve didn't work then your pot could rupture if it's a very old one, so it's a very important safety feature for older cookers. Modern pressure cookers have redundant safety features like pressure plugs to ensure safety.
Note that lack of a hiss doesn't always mean you are in imminent danger, usually it's a sign you don't have a good seal rather than a bad valve. I would suggest you track down the hiss, if it's coming from the valve then it's normal operation, if it's coming from around the lid then it's not sealed properly. When tracking the hiss down remember it's steam escaping and be careful not to burn yourself. If you can't see where steam is escaping you can use a big spoon to detect it rather than your ear, or a mechanic's trick is to use a short piece of hose or straw as a stethoscope.