Hiya. There have been a number of solutions suggested for this. Someone said when he took his dad out & sat him in the rear to listen for the rattle, it went. They put the small seat in the split seats down and it was moving/rattling. They popped something down beside it & it stopped.

Some have also said their rear light clusters squeak.

There's also a problem with the rear seat belt reels rattling against the bodywork but you'll have to pull aside bits of carpet to have a look. Again small square of foam fixed it.

Mine (which I've had from new) seems to be coming from inside the bootlid, where the black plastic is on the inside. I'll attend to it one day