The wedge cameras can also get a rattle in the mount, that's why they come with little stick on PU foam pads to put in between the camera and mount.
If it is the CPL filter i assume you could heat it up and maybe bend it in a little so it have a more firm grip on the camera, if it is the glass in the CPL i assume when properly tuned you could fix it in place with a tiny amount of hot glue on the very edge of the glass. ( do not use CA glue near camera lenses )
I have never heard of CPL filters rattling.