rear heat problem

on cold start up at idle i hear a gurgling noise in the dash..as the engine warms up, my rear fan blows cold air until i rev the engine up then hot air comes out.then let idle cold air....any ideas?? 2005 yukon 5.3

not that i can think of. my step dad's yukon had a similar problem before he got rid of it, but that was like a 98 model

took it to the dealer today they fixed it..they had to reprogram the heater computer!!!! i thought it would be the water pump or something..those computers run the whole dam car!!!