I have the same problem - and I believe I used the same pads. I think it’s the new clips, not the pads per se. The clips I took off the car (and discarded) had a coating on one side that I think held the pads better. I remember taking those clips off and thinking - what the hell is that coating for? Then I tossed them in the trash and didn’t think twice. About two weeks later, I got the rattling. I loaded some extra grease into that clip groove and it took care of the rattle for a little while. Then FCP sent me a set of LR brand clips to try, hoping they would have that rubber coating. They don’t - look identical to the other replacement clips.
I just ignore it - haven’t had the motivation to mess with it again.