A whistling noise heard specifically at night is frequently caused by nighttime temperature drops and shifting wind patterns interacting with your home's exterior structure or HVAC system. As outdoor temperatures fall rapidly after sunset, metal flue pipes, roof structures, window frames, and ductwork undergo thermal contraction, which can open small gaps or cause panels to shift and create aerodynamic whistling as nighttime winds blow across them. Additionally, if your HVAC system runs overnight, air pressure changes or a dirty air filter can cause supply vents or duct seams to whistle quietly in the stillness of the night. You can verify the source by checking if the sound correlates with wind gusts or stops when indoor vents are adjusted.
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