Randomly overheats

Truck randomly overheats, becoming more frequent.
Truck has appropriate level of coolant(mostly water at this point)
when truck is starting to over heat and I turn on the heater it will sometimes blow cold air and the truck will continue to overheat. If I toggle the air and or temp controls sometimes the air will heat up and the temp will return to normal.
when the temperature rises to overheat I can shut off the truck and restart in a few minutes and the temperature will drop back to normal before the system has had a chance to cool.
I feel that the issue may be an air lock, sensor or control valve. I have also changed the thermostat- no change.
sometimes I hear a hum like a fluid restriction under the hood near the firewall in the vicinity of the heater and control area.


Any advice would be greatly appreciated.