same problem with a 2020 plate CH-R! 4500 miles and has devolved a really annoying rattle. This occurs when going over slightly rough terrain or speed bumps!The sound seems to be on the drivers side at the back. Almost like the boot haven’t been fastens properly. Checked everything over and can’t find a particular problem but have headed all of your advice on hear and bent the wheel plated a little, put a cloth in the grove of the boot lid, checked the reg plate. Hopefully it might help. Really annoyed it is merely 3 months old.