Installed the new retaining clips and the rattle is gone!

I guess they just wear out after a while and lose their spring tension.

Thanks for all your help everyone!

 

Great!! Thanks for the update.
I was going to suggest Brakeleen and some fresh pad grease to rule out basic maintenance factors. Did this last night on my front right side and it took care of it.

 

An update here...

I did a proper bedding-in of the Endless CC-X pads on the freeway before going to Buttonwillow...they totally cleaned up the rotor surface and the steering wheel vibration is gone...they performed wonderfully at the track and I experienced no rotor issues even though it was 104F that day.

It must have been a combination of the brake ducting and the semi-racing pads, but it worked and that's all that really matters.

Looks like Axxiss Ultimates are going in the trash and Mazda stockers are going back on for street driving and I'm keeping Endless CC-X for the track. No more aggressive street pads at the track for me (they just get fried and take rotors out with them) and no more hybrid semi-track pads on the street (they just chew up rotors). I've learned my lesson. As soon as the Endless CC-X wear out I will look into Hawk DTC or Endless CC-Rg.