98 Ram heater acting wierd

It was a balmy -7 F this morning driving to work. I noticed that the heat was lukewarm when I stopped at a light, and nice and hot when moving. Temperature gage was right where it needed to be. I checked coolant level it was fine. Any idea what might be the problem?

flush heater core

Flushed the heater core. Flow was good. I still have the same problem and i get this gurgling noise when engine is still cold. Will the trapped air work its way out?