Sloshing sound behind dash again

Same Issue here as well, 07' 4.0L V6. Not sure what it is, It almost sounds like coolant flowing through the system for the first time

 

Did you run the heater core while doing the coolant change? Maybe there was air in the heater core/heater core lines even though when you burped the system and no air bubbles came out. Then when you're running the truck the coolant is flowing in/out of the heater core causing the noise. Just a thought.

 

I'm also hearing some sloshing right behind the dash when accelerating in my 2006 2.7
Going to do a full coolant flush and replace once the order arrives. About the head gasket leak, can't you test that with a combustion leak tester?