Diagnosis
The hissing you hear is almost certainly steam escaping from the vent tube (also called the vent pipe or steam vent), which is the small metal tube permanently fastened to the center of the lid. This is a normal and essential sound during the pressurization and processing phases. It indicates that steam is actively venting from the canner's interior. The pressure regulator (weight) sits on top of this tube during operation, and the hissing is the sound of steam flowing around and through it.
How to Fix It
This sound is expected, but you should verify everything is functioning correctly. First, ensure you are following the correct startup procedure: place the canner over high heat with the lid locked but the pressure regulator (weight) off of the vent tube. Allow steam to vent freely from the open vent tube for a full 10 minutes to exhaust all air from the canner. Then, place the correct pressure regulator weight (e.g., 5, 10, or 15 psi) onto the vent tube. The hissing will continue as the canner builds pressure. Once the correct pressure is reached, the regulator will begin to "jiggle" or rock steadily, typically 1-4 times per minute, and the hissing will become rhythmic. This is the sign that processing pressure has been achieved and your timer should start.
If That Didn't Work
If the hissing is excessive, erratic, or accompanied by steam leaking from under the lid or around the safety valve, there is an issue. 1) Check that the vent tube is completely clear by running a pipe cleaner or small brush through it; food or mineral deposits can cause irregular venting. 2) Inspect the sealing ring (gasket) around the lid for cracks, stiffness, or food debris, and ensure it is properly seated in its groove. A damaged gasket will prevent a proper seal. 3) Make sure the lid is correctly aligned and locked according to the handle alignment marks.
Heads Up
Never attempt to stop the hissing from the vent tube by covering it or reducing heat excessively; it is a critical safety feature that allows pressure regulation. Always use the manufacturer's specified pressure regulator weight for your altitude and recipe. If the hissing stops entirely during processing and the pressure gauge drops to zero, do not open the lid—this may indicate a loss of pressure, and you must restart the process from the beginning, including re-exhausting air for 10 minutes.