Is the knock a single knock or continuous, with engine rotation, as the pedal is depressed? If a single knock, does it occur without the engine running? If the latter it points to the mechanical clutch pedal end of the release mechanism, the gearbox end being hydraulic......... If it is the former, i.e. continuous during pedal depression, it points to a problem with the clutch release bearing and clutch diaphragm fingers inside the bell housing......