What is the reason for the metal rubbing sound in the ignition of cold car?

The causes of metal rubbing sound are: 1, and the sound from the valve: when the cold car is started, the water temperature and oil temperature are not high. If the oil viscosity and lubrication are not enough, the top cylinder cannot be adjusted, so there will be metal friction when the cold car starts. 2. There is something wrong with the hydraulic tappet: the oil pressure is too low or the hydraulic tappet itself is worn. Once the air enters the tappet, there will be metal friction when the car starts. 3. Wear of the valve rocker arm leads to excessive clearance or carbon deposition: a dirty throttle will also make a metallic sound when the cold car starts. 4. Improper assembly of engine front cover: If the output power increases when the car is cold, the engine noise will increase accordingly. This is a normal phenomenon. If the noise is too loud, you can reflect this situation when doing maintenance. 5. Carbon canister solenoid valve works: The metallic sound in this case is normal and can be used continuously. But if the noise is too loud, you should go to the 4s shop for inspection.