DIAGNOSIS A constant, loud hissing means steam is escaping during the pressure cook cycle. It's not normal. While a quiet, intermittent "pssst" from the pressure release valve is okay, a steady, loud stream indicates a seal problem.
FIX Turn the pot off (hit "cancel") and let it depressurize completely. The main fix is almost always the sealing ring. 1) Remove the silicone ring from the lid. 2) Wash it well—hot, soapy water gets rid of oils that can cause slippage. 3) Dry it thoroughly, then make absolutely sure it's seated evenly in its groove around the entire lid. A common mistake is one section popping out of the channel. 4) Also, check the steam release valve on top of the lid. Make sure it's set to "Sealing" (the arrow points to the little closed lock icon) and not "Venting." The valve should move freely; clean any food bits from underneath it.
IF THAT DIDN'T WORK Look at the float valve. If it's not clean or has debris under it, it might not be rising all the way, which can allow steam to escape around its stem. Pull it straight up to remove it, clean the stem and the hole, and drop it back in. Ensure it's not stuck.
HEADS UP A loud, continuous hiss means your pot isn't building full pressure, so cooking times will be way off and recipes will fail. Don't ignore it. Always start with cleaning and reseating the sealing ring.