Firstly there is a problem with the belt / pulleys
That spring should not be ossillating like it is in the video
Secondly the metallic sound is the PTO clutch
Hammer some plastic wedges between the slot in the clutch and the anti spin peg
Some mowers had plastic / rubber sleeves to stop the clutch ratteling while others had a light spring to keep the clutch tight against the face of the peg .