It turns out that while I've certainly fixed the loudest and worst of the rattles I still have a feint metallic sounding rattle from the rear somewhere when hitting certain kinds of bumps. I suspect I didn't hear it before because of the racket the D Pillar trim panels were making. Very annoying and I have no idea where to look now!

When I had the left rear trim panel off I checked all the various modules, gadgets and wiring behind it and did't find anything that sounded the slightest bit rattly or loose. This new one sounds quite metallic, like metal tapping against metal but also quite feint...

I've noticed when I'm outside the car if I tap the rear light clusters they rattle, but I can't believe that would be it?

Any suggestions anyone?

Heat shields underneath

To add a bit more to EYorkshire's suggestion above, they are held on with springy washer things that I found need pushing on a little more, hey presto, rattle gone. I could reach them without needing any lifting of the car. 2016 BMW X3 30d SE in red
MY2013 SD4 XS Firenze Red, 19" 10 spoke contrast paint wheels, Signature Strips, Grand Black Lacquer trim, Cold Climate Pack - gone!
2007 TD4 S Stornoway Grey - gone!

Thanks for the tip fellas, that's one rattley sheet of metal. I found mine was fixed on by two of the spring clips you mention, neither of which were pressed on as far as they'd go, and also by two or maybe three 24mm flat pressed nuts. These were all tight as far as I could check because the front most ones were under some chassis member or something.

I also found a couple of pipes/cables that run from behind the LH rear wheel arch liner and were very easily tapping on the chassis. There's a hole in the body near them almost implying there should be a clip holding them. I wrapped a bit of tape around them and cable tied them so they're just away from the chassis.

Hopefully one of these two exercises will have fixed it. I like the exhaust idea because I'd already noted the rattle was hardly there at the start of a journey and got progressively worse as a journey continued etc.

I'll let you all know

Well, it wasn't the heat shield or the two pipes in the left wheel arch. But, whatever it is is absolutely heat dependant. I don't though know if that's engine, exhaust, interior or brake heat only that the rattle isn't there when cold and progressively worse once started. It certainly sounds metallic, but only light metallic such as an ally plate vibrating against something else and it is activated by small bumps and ripples of which there are of course billions on our wonderful UK road surfaces.

Last night my wife kindly offered to lay in the load area (wash your minds out now) and have a good listen and she determined the noise was loudest coming from the vent above the rear right wheel arch. I guess this would make sense as to me it's always sounded as though it were from the left so that's where my efforts have been concentrated. She managed a number of contortionist movements to triangulate the sound and is adamant that it's from the right and loudest by the vent - which of course if would be as it's an opening to behind the trim panel.

However, what she can't tell of course is if the sound is being generated by something inside or outside the car. So, I guess the next step is to pull the right trim panel off and see what's going on behind that one.

It's a shame I can't just hand the car to my dealer and trust them to do a job properly - still, the plus side is that I've already found and fixed many rattle sources and so I'm pleased about that.

Just another update in case anyone's still interested

Took the RH panel off to find the DAB tuner (metal) bolted on top of some other audio unit (Also metal, I suspect the amplifier)? bolted to a metal plate bolted to the car. I won't say some of the bolts were loose but let's say they weren't particularly tight either and so I tightened them all up. Only broke one plastic clip with this panel but not an issue now I have spares... £3.20 for one plastic clip!!!! While I had the panel off I took the opportunity to add some silicone around the plastic clips on the RH D pillar trim so they can't rattle either.

I think from a drive later in the day, although I'm hesitant to suggest it will be permanent, that I've finally nailed that rattle. Now there's just a cracking noise when hitting bumps from the LH side of the dash to fix.

Onwards and upwards.

It's so much simpler when like me, your hearing ain't like it use to be I may have more rattles than you but falls on deaf ears .