i was losing heat so i put a new thermostat in. still losing heat mostly when i'm giving it gas. seems like it blows heat whenever i'm coasting or giving it barely any throttle. every once in a while it sounds like water rushing behind the glove box where the heater core is located. could that be the problem? it's not leaking or anything. i just drained out the old antifreeze and put new stuff in when i changed the thermostat. thanks.

btw it's a 90 f150 4wd 5.0

 

If you heard the water moving , you have air in system. check water lavel and fill up if necesary. Also check for leaks.