It sounds like different cars have different sources of rattle and solving one problem merely reveals another. I've had rattles from behind the boot trim of my 06 GTI for a long while, but one in particular is loud and triggered by the slightest movement, so is almost constant. It transpires it's one of two microswitches connected to the wiper motor. They are fixed to the bodywork, but not securely, so removing their freedom stops the rattle. Sadly it's only proved a temporary solution, as the noisier one keeps breaking free thanks to the potholes and cobbled roads around here. I'll keep working on it.
I hope that helps someone.