I took a 2018 Roubaix demo bike out today and had that exact rattle coming from the handlebar stem area.
After two hours of trying to identify the source of the noise I eventually pressed my thumb down on the pivoting cup at the base of the headset where it meets the top tube and it stopped.
In summary, the plastic cup is vibrating against the top tube on uneven, rutted surfaces. Big bumps don't affect it.
I then tried to tighten the grub screws but the were all solid so no adjustment possible.
This is clearly a design fault so I am now having second thoughts about buying.