What is the reason for the knocking noise during cold start of Passat?

[NatalieDella]

Passat's knocking noise during cold start is caused by the lack of lubrication in the starter's internal gears, leading to sticking and affecting the timely return of the gears. Low engine speed: When the flywheel engine starts, the starter rotates to drive the flywheel and start the engine. At this time, the engine speed is driven by the starter, so the engine speed is lower than the starter speed. When the engine speed after starting is higher than the starter speed, manually operate the ignition switch to turn off the power and shut down the starter. Starter lubrication failure: Due to the internal dirt of the starter and the failure of the lubricating oil, the flywheel engine drives the starter to operate at its own speed. Because of the speed difference between the two, the starter will produce the heard "dirty" sound. When the starter and engine are in a hot state, this noise will stop, but there will be severe noise every time it starts. To remove this noise, the starter can only be cleaned and maintained or replaced, but the cleaning and maintenance effect will not be very good, and the noise will soon return.