CLUNKY REAR END. (not talking about the wife.)

[whitehawk]

Had my truck for just over 3 months now and ever since i bought it i have had a rattling or clunking noise when i go over bumps or potholes. It's not the usual metallic rattles or worn bushes noise but sounds more like a socket set clunking in a plastic box. I have checked all the rear bushes exhaust system and removed all items from inside the back, removed the spare and jack, tried folding the seats, made sure the tailgate and doors are tight but it is still the same. Any ideas or suggestions please? (Apart from turning up the radio.)