What is the reason for the metal friction sound when releasing the accelerator?

[McAnthony]

It may be due to noise from the engine timing chain or a malfunction in the timing sprocket. Below is relevant information about the engine: Introduction: An engine (Engine) is a machine capable of converting other forms of energy into mechanical energy, including internal combustion engines (reciprocating piston engines), external combustion engines (such as Stirling engines, steam engines, etc.), jet engines, electric motors, etc. For example, internal combustion engines typically convert chemical energy into mechanical energy. Expansion: The term engine can refer to both the power generation device and the entire machine including the power unit (e.g., gasoline engines, aircraft engines).