2005 GMC SIERRA 2500 HD COOLANT LEAK

[ATOMICUGLY01]

I am sure this has been asked before, but I was unable to locate the info I am chasing.  I have a 2005 GMC SIERRA 2500 HD 6.0l VORTEC, that is leaking coolant from what appears to me to be from under the center of the radiator.  The part I am having a hard time getting my head around is it only leaks when there is no pressure on the coolant system.  I was changing the oil on my truck when I noticed the coolant spot on my driveway.  When I filled the coolant overflow and squeezed the upper radiator line coolant dripped from the drivers side under the radiator.  All of the lines were replaced last summer as was the radiator.  When I started the truck to warm it up, the drip stopped.  I ran the truck for another 15min hoses got warm pressured up and the drip stopped.  Any idea where to go from here?  

Thanks in advance