short length of garden hose ( or heater hose) to your ear .. other end to valve cover ... move hose along while the motor is running ... you should be able to narrow it down ( loudest clacking )
then stop engine , remove valve cover , move cam till the lobes are UP for the bad cylinder ...... then press on the tops of the 2 followers ( one will move easy ... the bad one) .....
setup for belt removal .. TDC , cam locked, IP locked ... remove CAM , swap lifter (s) , replace cam, reinstall BELT , retime and adjust pump, reinstall valve cover ...
OR is it just Knocking from a drained injector line ( run at fast Idle for 5 MINS and see if it goes away )