Car heating system

Make sure the coolant is full. Make sure the engine is running at a normal operating temp. Next, with the engine running, hot, and the heater on, feel both heater core hoses. Both should be hot. If only one is, flush the heater core. If both are hot, chances are there is a problem with the blend door. If the knocking sound is under the dash, the blend door is my first guess. If the noise is coming from the engine, that is a different situation and most likely involves internal engine bearing issues.