What Causes the Sound Under the Hood After the Car is Turned Off?

[VonSara]

It is normal to hear a popping sound after turning off the engine. Pay attention to whether the sound comes from the bottom of the vehicle. If so, it is the sound made by the vehicle's three-way catalytic converter as it cools down. This is the sound of the muffler expanding and contracting due to heat after the engine is turned off. Many cars experience this phenomenon, and it is completely normal. Since the exhaust pipe reaches very high temperatures during engine operation, manufacturers install heat shields between the exhaust pipe and the vehicle chassis to prevent excessive heat from causing the rubber and plastic components in the chassis to age prematurely. The high heat causes the heat shield to expand, and when the engine is turned off, the heat shield cools rapidly. The rapid alternation between hot and cold temperatures leads to thermal expansion and contraction at a fast rate, causing the heat shield to deform and produce the sound. It is similar to the sound made when repeatedly bending a stainless steel sheet back and forth with force.