Silverado :: Sound Almost Like A Rattle When Driving On Gravel Roads

Ok, I have a noise, rattle, clunk that is bothering the heck out of me. I know it is not the famous clunk from the yoke or the pads missing on the leaf springs. I have even removed the Tailgate thinking that was the issue, but no it was not.

When driving on gravel roads or if I hit small bumps in the road I hear the noise and it sounds like it is coming from underneath and towards the rear of the truck. It is hard to explain the sound, but it almost sounds as if there was something loose and the metal is hitting again the frame?? I have 126000 miles on it, so I figured I would replace the shocks, they needed it and that did not work, it is still there.

Now the twist, when the temp outside gets below 60 the noise starts to fade away or I have to run over a bigger bump to hear it, and below 40 it almost does not exist. Now the hotter it gets out, the more I hear it and during the summer months it is really annoying. I have also looked underneath and could not find anything that looked cracked, broke or loose.