my truck seems to be making a clunking sound in the rear when i go over bumps or turn while i am going kind of fast. What could this be?

 

Our's have different rear suspensions but I've got something simalar, I think. It's one of those things that I can't rtemember if it's always done it. Mine could be normal sounds with the rear LLS. Nothing loose I can find and nothing weird about how it drives. I've wondered if maybe the fuel tank was slightly loose and it could be banging around when jolted good like over a speed bump. That's not a good thought.......