A whistling sound is generally caused by air or gas being forced at high velocity through a narrow opening, a restriction, or an irregular gap. In residential settings, common culprits include a severely clogged air filter that forces the HVAC blower fan to pull air through tiny gaps, partially closed supply registers that restrict airflow, or loose joints and unsealed seams in your ductwork. Outside of home systems, wind blowing across sharp architectural edges, pipes, or chimneys can also generate aerodynamic whistling tones. You can easily diagnose and verify the cause in an HVAC system by replacing dirty filters, opening all room vents fully, and inspecting accessible duct connections for gaps or leaks.
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