If your vehicle is showing hissing sound from the exhaust area, here are the most likely causes and the parts you'll need to fix each one.
The gaskets sealing the connections between exhaust pipes can degrade, burn out, or fail due to loose hardware, allowing high-pressure exhaust gas to escape with a hissing sound.
If the exhaust system was recently serviced or replaced, a loose exhaust clamp or an improperly seated slip joint will allow exhaust gases to hiss out under pressure.
Rust, road salt, and constant heat cycling can create tiny pinholes or cracks in the exhaust piping, resonator, or muffler, causing a continuous hiss while the engine runs.
A cracked exhaust manifold or a failed manifold gasket near the engine block can create a hissing or ticking noise that echoes down the exhaust area, especially under load.
A clogged catalytic converter creates severe exhaust backpressure, forcing gases to hiss out of upstream joints, flex pipes, or the converter shell itself. This is usually accompanied by a severe loss of engine power.