I think I may have found the problem. There are multiple skins around the silencer and most of the outer skin has rotted away. Above the silencer there's a big piece of outer skin that's only attached at one side so it's flapping around. Guess when it's doing a regen it's vibrating at just the right frequency to make it rattle like mad . I'm going to remove the corroded outer skin tomorrow and see how it goes. Fingers crossed if this works it won't cost a penny to fix.
That didn't 'fix' it on mine. The outer skin was indeed loose and flapping around but once I'd removed it, the rattle was still there during a regen.
The funny thing is, when the outer skin of the back box was loose, the main dealer commented on it twice on two separate occasions during servicing. When I asked them if it was problem, they said 'well it would probably fail an MoT'. Since the car was not of MoT age at the time, it never went any further.
Subsequently, the rattle during regens reared it's head and one of my initial thoughts was that it was the loose outer skin, so I removed it - no change at all. But now the back box looked a lot better and it was never raised at the main dealer again - it even passed the MoT when it became due.
The rattle during regen was mentioned a few times and each time it went to the dealer, they said they could find nothing wrong, although granted they probably never experienced it themselves.
My money is on it being an internal baffle or something in the back box. Although just because the noise is emanating from the back box might not mean it's the back box itself.
I learned to live with it, and actually grew to like it. Nothing ever dropped off and indeed the structural integrity of the exhaust system was checked numerous times by the dealer.
Without replacing components I don't think you'd get to the bottom of it, unless somewhere, someone else who had this issue found a fix ?