Ok..cars been and had its rear brake recall. I asked them to look at this rattle and they cannot find anything obvious. Probably because the car was only with them for a pair of hours and as the noise only seems to occur when left for a much longer time to fully cool down, that's no surprise.
I've watched Youtube videos of failing DMF's and the symptoms/noise seem different.
I'll just have to put up with it and act if/when it gets worse or I find a definate cause. At least I've brought it to the attention of Mazda so if it all goes pear shaped, I've at least made them aware.