When a furnace produces a whistling noise right as it starts up, it is usually experiencing an immediate airflow restriction or a pressure imbalance in the ductwork. The most common cause is a dirty or clogged air filter; when the blower motor kicks on, it immediately demands high airflow, and if the filter is choked with dust, it creates high-velocity suction that whistles through the filter slot or return grilles. Another possibility is that some of your supply registers are partially closed, forcing air to squeal through narrow openings. You can easily troubleshoot and verify this by checking and replacing your air filter and ensuring all room vents are opened fully. If the whistle persists, inspect the blower compartment for loose panels.
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