I've tried glovebox open, empty, bit of card wedged between button and centre console. If i poke my finger in the aperture for the glovebox lock and pull it to one side it improves a little. I think it's exacerbated by a genrally quiet cabin. As you rightly say, it might be a red herring and be from somewhere totally different. It does have a loose spring/washer metallic sort of sound, and only on bumpy roads, which unfortunately is most roads seemingly