Land Rover says that some 2001 Discovery Series II and Range Rover 4.0 and 4.6 models may emit a knocking sound from the bell housing when the starter motor engages.
The noise dissipates once the engine starts.
One likely cause of the noise is the starter motor pinion making contact with the flywheel balance weights. Installing new-design, lower-profile weights from a kit, part # LBB000550, onto the flywheel should eliminate the condition.