Probably the blower motor resistor, check blower motor at the same time
Replace heater motor resistor.
The heater motor speed resister is burned out.
the heater resistor needs to be replaced it is mounted in the heater box next to the blower motor
try replaceing the switch that controls the low to high for heater It's gonna be the blower motor resistor (near the blower motor).
The blower motor speed resistor is burned out.
Most likely and bad blower motor resistor.
The blower motor resistor is bad and goes to high fan.
I had the same issue with my Cherokee Sport 4.0 until I had the heater core flushed. After a couple trips once flushed, high pitched sound discipated.
1. heat control linkage disconnected or broken or vacuum source problems(low vaccum source or broken vacuum lines) 2. heater unit doors stuck or non functioning heater door actuators
The blower motor speed resistor is burned out.
Normally when a heater blower motor only works on the High setting, it is caused by a defective Blower Motor Resistor Pack. The fix is to, Replace it. In very rare instances it can also be a defective blower motor speed control switch, but rarely.
It is the resistor, replace it.