Faults and Technical chat for the Citroen C4 Cactus
The 'metallic rattling' suggests pinking, which means either the plugs are faulty (happens a lot with modern engines) or the timing is out?
C5 Aircross PureTech 130 S&S Flair, Tijuca Blue/Black Roof, Metropolitan Grey ambience
Also can be attributed to the fuel having too low an octane rating than is required for the high internal pressure in the cylinders, particularly relevant for engines having turbo boost. See https://en.wikipedia.org/wiki/Engine_knocking