I am guessing your rattle is the plastic that GM used on these rear deck plates! I get a rattle also when I play any music with deep bass sounds!
Many have this same problem because GM decided to put plastic instead of some other material that would not rattle! I think GM had over done it a little too much with the plastic!
If your rattle is really bad, you may have loosen something up by high volume settings , who knows? I never had this issue with any of my other vehicles, current / past except this car but again my other cars don’t or didn’t have any plastic deck plates!
I am sure there are some Guys here that could give you some advise on insulating and the use of some materials to help eliminate / reduce the rattle!
Good luck!
