If you discover a knocking sound from the extruder, it means the filament is not fed smooth through the nozzle. Please try the following things:

1. If the gap between the nozzle and the print bed is too narrow, the filament will not extrude.
   -Increase the gap between nozzle and plate to 0.4 mm.
   Tips: A regular writing paper is about 0.1mm thick, fold a paper twice to make 4 layers. This will be a good measurement for 0.4 mm.

2. If the temperature is too low, the filament can not be melted down quickly enough and squeezed out smoothly.
   -Increase the temperature by 5-10 °C to see if the filament is fed more smoothly.
   Tips: Recommended printing temperature is 215-225 °C.

3. If the filament is tangled in the filament roll, or the filament roll is stuck in the holder by some reason, the extruder can hardly feed the filament.
   -Check the filament roll and make sure the lid is not too tight.