If your fire pit is hissing, it's likely because there's water in the gas line. This can happen if the gas line isn't properly sealed or if condensation has built up inside the gas line. To stop your fire pit from hissing, you'll need to remove the water from the gas line.
One way to do this is to turn off the gas supply and then disconnect the gas line from the fire pit. Once the gas line is disconnected, open up both ends of the gas line and allow any water to drain out. You may also want to use a paper towel or other absorbent material to soak up any remaining water inside the gas line.
Once you've removed all of the water from the gas line, you can reconnect it to your fire pit and turn on the gas supply. Your fire pit should now be working properly without any hissing sounds.