You don't want to lower the resistor or your hysteresis will be huge.
A much better solution is to low pass filter the 12V power supply for the comparator with a big capacitor 500-1000Uf. That will stabilize the voltage so the fan noise on the 12V supply cannot get to the comparator.
100 uf across the PTC, another on each of the setpoint potentiometers center tap should filter those signals further reducing the noise.
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