It sounds like you are getting just a little flow through the heater core for whatever reason. Either the valve is not completely open or there is a clog. Or there is air in there, but I can't explain why you can't get the air out. If there is a slight clog then there is not enough flow to force it out maybe?Corrolla heater only blows hot air after turning fan on then immediately goes cold. If I turn fan off for a while, blows hot air then immediately goes cold again. I can hear coolant occasionally swishing through dashboard. Recently did flush and found not clogging in any engine orifice or heater core tubings.