Almost since new, in March, my car has had a rattle from the dashboard cubbyhole area, to the right of the steering wheel. I had convinced myself it was the cubby lid that was rattling, as it seemed to quieten when I held it. Until today, I hadn't done a proper investigation, but after I removed the cubby lid, (open it and squeeze the sides together, quite hard!), the actual cause became apparent. The panel in the photo sits under the dash, below the cubby, between the driver's knee airbag and the footwell light/OBD socket area. It should be secured by the T20 screw at the top of the photo (nearest to the driver) and a tongue, which slides into a clip on the bulkhead at the bottom of the photo. The person who assembled mine had missed the clip, so the panel was bouncing up and down and, err, rattling. Unscrewed it, assembled it properly and, very satisfyingly, the rattle has completely gone!
I can't believe mine is the only car this has happened to, so, if you've got an unidentified rattle in the cubby area, it might be worth taking a look.