No heat

I have no heat? I changed the t-stat, pulled the hoses off the heater core and put a water hose to the heater core and water comes out, so it is not blocked. There seems to be little flow to the heater core cause every now and then I can here a trickel in the core. There is worm air if I keep the fan on low, but on any other speed it blows cold. I am sure the flap that moves from cold to hot is moving, I checked that too. The truck does not over heat. I am lost on what to do next. Any suggestions?