Loud Knocking noise from my Renault Scenic desiel 1.9 Reg 03?

In the last day I have heard a loud knocking sound on the above vehicle, start up I have removed the pulley belt and the sound persists. I have yet to remove the cam cover. There is no indication of a dropped valve. However, could there be a stuck valve. I have seen reports that it may be something to do the variable valve timing the ‘dephaser’