If your vehicle is showing hissing sound coming from the radiator or hoses after turning off the car, here are the most likely causes and the parts you'll need to fix each one.
The radiator cap is designed to hold a specific amount of pressure. If its rubber seal degrades or the internal spring weakens, it will prematurely vent hot steam and coolant to the atmosphere or overflow tank after engine shutdown.
Rubber cooling hoses degrade over time and can develop tiny pinholes, or hose clamps may not have been tightened properly during recent repairs. Pressurized steam escapes through these small gaps, creating a hissing noise.
Modern radiators typically use plastic side or top tanks that become brittle from constant heating and cooling cycles. Hairline cracks open up under the high pressure of a hot engine, releasing hissing steam.
The plastic expansion tank (or its pressurized cap) can crack with age. When the engine shuts off, 'heat soak' temporarily raises system pressure, forcing steam out of these cracks.
If the cooling system was not properly bled after a recent coolant flush or part replacement, trapped air pockets will expand rapidly when hot. This forces the system to vent the excess pressure, causing a hiss.
Many vehicles use plastic thermostat housings that can warp or crack over time. Pressurized coolant escapes as steam near the upper radiator hose connection.