[stuart bowes]

I thought the same (at certain bass frequencies in my case) but it turned out to be the rear door trim, the plastic strips surrounding the door opening lever. In my case I reupholstered doors anyway and just put them back tighter but could also achieve by flipping over door card and wedging something under the plastic on the other side

Also the metal rods for the door lock tend to rattle a bit, I had to stick some thin foam strip behind them

May or may not be the same in your case