Javier:
The factory mudguard does not appear to be loose. I tried to move it and its screwed on pretty solid.
The rattle I am experiencing is at certain RPMs when there is just the right amount of vibration.
Under acceleration, I do not hear this rattle.
The problem is not if it's loose or not, but if they have put the screws the other way round. If you check again and see the heads of the screws, it means their long part is inside and too close to your tyre. If they touch the tyre when you turn and the rear whole flexes, you can have your tyre damaged. And of course a nice rattle. In my case, I had a rubbing noise since the very beginning, plus a huge rattle while Turing Sharp. And I discovered I had the screws improperly installed, and one of them carving my tyre permanently, making a channel in it.
Might not be your problem. But worthy to double check again! I don't know if you read already this, but just in case I put again the link to my experience
https://www.speedymarmots.com/works-5-properly-re-installing-the-rear-mudguard/.
Regards
