GM-GMC: Coolant loss, head gasket problem, coolant loss

Question
I have a 2005 GMC Sierra w/5.3 engine
It loses coolant but there are no signs of external leaking
I did have to replace the water pump recently and noticed black sooty looking stuff in the old coolant
I also get who sounds like detonation when i start it cold but that goes away after it gets to operating temp
could this be a head gasket problem?

Answer
Typically the first place to look for a concern like that is the water pump gaskets.  They are prone to leaking without any signs on that year/make/model of truck.  As for the detonation, it is likely piston slap again not abnormal for that truck, piston heats up, expands and stops making noise.
Hope this helps