The hissing noise after engine off is usually caused by the cooling system. As the engine cools down, the metal contracts and the coolant contracts as well. This contraction creates a vacuum in the cooling system, which draws air in through the radiator cap. The air mixes with the coolant and creates a bubbling sound, which is what you hear as a hissing noise.
Other potential causes of a hissing noise after engine off include:
* A loose or missing radiator cap
* A leak in the cooling system
* A faulty thermostat
* A water pump that is not circulating coolant properly
If you are concerned about the hissing noise after engine off, it is best to have it checked out by a qualified mechanic.
