A high-pitched sound coming from a furnace is typically caused by mechanical friction in moving parts or aerodynamic whistling from airflow restrictions. The primary mechanical cause is a failing blower motor or draft inducer motor with dry, worn-out bearings that emit a sharp squeal or whine as they struggle to spin. The primary aerodynamic cause is a severely clogged air filter or blocked return vent that forces air through narrow gaps at high velocity, creating a loud whistle. To diagnose the issue safely, turn off the system and inspect the air filter; replacing a dirty filter often resolves whistling instantly. If the high-pitched squeal comes directly from the motor, professional repair or motor replacement is required.
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