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The speeds are controlled by a resistor pack. Changing the resistor pack may restore full functionality. Volvo heater fans are notorious for squealing. Usually they are run at a speed that doesn't squeal. If it squeals at one speed, sometimes a different speed won't squeal. The resistors seem to eventually fail. They are fairly cheap to replace.
possible bad connection at motor or bad ground but if it has fully electric controls you might need a scanner to find out the codes oh sorry just checked its a 92 so i would go for connections at the motor first or try a good known motor it could have a bad armeteur inside
make sure it is full of coolant, next check for a bad fan relay, to see if fans are ok! run a wire from positve post on battery to positive on fan motor, ground other side if need be. if fans do not work they are bad, if they do work, check relay to fan and coolant tempurature sensor if needed.
Run 12V directly to the motor and see if it will run, if it dosent than the motor is bad, if it does than the resister is bad, the resister makes the blower run and also regulates the speeds for the blower, but chanses are if it was making a high pitch squeel before it quit the motor went out.
It's very possible the fan motor has quit. The squealing noise is caused from motor bearing failure. As the bearings wear out or dry up the motor is harder to turn and require more current to run the motor. On lower speeds, the motor runs off a series of resistors that reduce the amount of voltage and current so the fan runs a lower speeds. If the bearings are worn, there is not enough voltage or current to turn the motor and that'sprobably the humming noise your hearing.
Good observation by the way.
Let me know.