Heater not working properly

Turn control knob all the way to hottest, cool air blows out vents. When I turn defrost on, A/C compressor kicks on and blows ice cold air. Vehicle temperature is normal, so that should rule out thermostat issues or low coolant, correct? When switching between hot and cold or vents and defrost, I hear no noises from blend door actuator area. I don't want to buy a heater core and that not be the problem. Any ideas how to narrow things down further?