This could be a few things such as anti roll bar drop links joint wear (check by disconnecting one end and checking for a lot of play in the joints), perishing of the strut top mounting since this is a bush held in rubber, or more likely a common fault is failure of the shock absorber (strut). To check the top mounting and the shock absorber needs the strut removing and the coil spring removing.
Our 2015 C4 developed a metallic knock at just over 4 years and 25k miles at the front left suspension. Disconnecting the anti roll bar drop links did not show any play in them so I had to remove the left strut. Once the coil spring was removed I found the top mounting was in good condition but there was side ways movement in the shock absorber shaft which made the metallic knock. Apparently the shock absorbers are poor quality and Citroen have changed them so if one needs replacing both sides have to be done at a cost of £67 for each side from Citroen (other possible suppliers were a lot more than this). I also replaced the complete top mountings at £110 each side to save the possibility of these needing doing in the near future. These prices were June 2019. It took me 7 hours total to do both sides in one go since the windscreen scuttle has to be removed to access the strut top nuts, so a dealer might take 3 or 4 hours.