I just bought a used 04 4runner. It is a v8 limited edition. There is a constant metal rattling sound from the back. When it runs a very low speed, there is no noise, but as soon as it picks up speed, the noise becomes louder and constant. I check the tools, the hatch, the spare tyre, license plate, towing part. Nothing is loose. It sound like a thin metal piece rattling. Any suggestion?