An HVAC system producing a whistling noise is usually suffering from airflow restrictions, air leaks in the ductwork, or improperly adjusted supply registers. When your indoor air filter is clogged with dust and debris, the blower fan struggles to pull enough air, creating a high-velocity suction that whistles through small gaps around the filter slot. Similarly, if supply registers are partially closed or blocked by furniture, air forced through the narrow openings creates a whistling sound. Additionally, loose joints or gaps in your return or supply ductwork can cause air to whistle as it escapes under pressure. You can easily diagnose and verify the cause by checking your air filter, opening all room vents fully, and inspecting accessible duct connections.
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