Knocking!!!

I currently own a 2000 'W' Mondeo TD LX. This vehicle was originally bought as a fleet car but I have owned it now for about six months. My problem is, that when driving over (some) uneven roads there is an
awful lot of knocking/vibration from the rear of the vehicle. I am almost certain that the noise comes from the inside of the vehicle but cannot be 100% sure of this. I have checked the plastic panels on the inside of the boot and tailgate, simply by 'shaking' them. The spare wheel and tools are firmly screwed to the bottom of the car and the hardboard panel that covers them is also secure. Even on the outside of the vehicle everything seems to be in order. I have even gone to the extreme of lying in the back of the car, while being driven by my wife, The only conclusion that this drew was that nothing was loose but I did find that by holding the tailgate firmly down, with the scooped out handholes, the knocking stopped. This puzzled me as the tailgate closes firmly and when the vehicle is stationery it has no movement in it what so ever. Is this problem recognised as a Mondeo fault/something that can happen over time? I have asked friends with other Mondeo's if they have this problem, but no one does. While I can live with this the knocking/vibration is extremely distracting when driving. Any help that you could offer would be much appreciated.
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