Why is there a whistling sound?

A whistling sound is generally produced when air, gas, or steam is forced at a high velocity through a narrow restriction, an irregular gap, or across a sharp edge. In a residential setting, a whistling noise frequently originates from your forced-air heating and cooling system, often caused by a severely clogged air filter that forces the blower fan to pull air through tiny gaps, partially closed room registers, or unsealed joints in your ductwork. Outside of home HVAC systems, environmental whistling can be caused by nighttime winds blowing across architectural features, roof structures, window frames, or plumbing vent stacks. You can easily diagnose and verify the source of the noise by checking your air filter, ensuring all room vents are opened fully, and listening to determine whether the whistling stops when your HVAC system is turned off.