I found 4 possible trouble areas.
1) I think KK's rattle was caused by the rear speakers rattling against their mounting brackets. There are only 2 screws holding each speaker to their bracket and it was possible to move the speakers side to side on the unsupported sides of the speakers, if that makes sense. Mazda should have added a gasket under the speakers or chosen speakers with 3 or 4 mounting screws. I added some double sided tape to the rear of the speakers and reattached them with their 2 screws.
2) Another possible cause was where the speaker bracket mounts to the rear support beam. There were sharp corners on the bends in the bracket that were squeaking against the support beam but a fair bit of force was required to cause the squeak. I isolated the bracket from the support beam with a cardboard gasket just in case.
3) Another possible cause was the 4 white clips holding the vertical rear plastic trim to the rear support beam.
4) Mazda US has released a TSB indicating that the noise was coming from loose screws holding the rear console latch. This wasn't the cause of the noise in KK's car. The screw's weren't mega tight but there was no play in the latch to cause any noise.
To get the rear trim piece off the car is fairly straight forward. It requires removal of the centre console which was a bit of a pain as it has 2 very firmly fitting covers over the screw holes. Once the screw covers were off the console it was only a matter of removing the 5 screws used to attach the console, the wind blocker, the front facing plastic roll bar trims, the sill plates, the seatbelt covers and a lot of clips holding the rear trim piece on.
I got everything back together and his car is rattle free. Bliss!
Hope that helps someone.
. A bit of black stubby cooler foam fixed that until the 10,000km service.
