If you discover a knocking sound from the extruder, it means the filament is not fed smooth though the nozzle. Please try the following things:
1. If the gap between the nozzle and the print bed is too low, the filament will not extrude.
Increase the gap between nozzle and plate to 0.4 mm.
Tips: A regular printing 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 cannot be melted down quickly enough and squeezed out smoothly.
Increase the temperature by 5-10 C degree to see if the sound disappears.
Tips: Recommended printing temperature is from 215-225 degrees Celsius.
3. If the filament is tangled in the filament roll, or the filament roll is stuck in the holder by some reasons, the extruder can hardly feed the filament.
Check the filament roll and make sure the lid is not too tight.