DIAGNOSIS The cooker isn't sealing. Pressure is escaping through the pressure regulator (the weight on the vent pipe) instead of building up behind it, which is why it keeps hissing.
FIX Turn off the heat and let it depressressurize completely. First check: Is the sealing ring (gasket) properly seated in the lid's groove? Even a small section popping out can cause a leak. Remove it, clean it, and put it back evenly. Second, make sure the lid is locked perfectly. The arrows on the handle and pot should line up, and the handle should feel fully engaged, not loose.
IF THAT DIDN'T WORK Check the pressure regulator weight and the vent pipe it sits on. Make sure the vent pipe (the little metal stem) is clear of any food debris. Also ensure the regulator weight itself is clean and can rattle freely if you shake it. A clogged vent will prevent proper pressure control.
HEADS UP A constant, loud hiss means it's not cooking under proper pressure. Your food will take forever and likely burn on the bottom. Don't just turn up the heat to compensate; fix the seal first.