Changed the screen and I can hear a rattling sound

I haven't had to open up my 6 Pro yet, so I'm feeling lucky in that regard, but after viewing the repair instructions there are only a couple of things that can possibly be rattling after a screen replacement.

I'm assuming you basically followed the iFixit guide. It could be if you didn't get the fingerprint reader glued down that it could be rattling, but I don't give that a high priority on the list of possibilities. I'm thinking that if you didn't get the screw put back in, or put back in correctly, on the display cable bracket, that it could have then fallen out and is now rattling around inside the phone.

Yeah, you're not the first person to run into the protective liner inside the camera peephole. I specifically went into the guide and added a cautionary note about that the last time someone had that exact same problem.