The whistling noise you are experiencing in your car's AC system when in fresh air mode, but not in recirculation mode, could indicate a potential issue with the air intake or cabin air filter. Here are a few possible causes:
Air intake obstruction: The whistling sound could be caused by an obstruction in the air intake system. Leaves, debris, or other objects may have entered the air intake ducts, causing the noise when fresh air is drawn into the system. Check the air intake area under the hood for any blockages and remove them if present.
Damaged cabin air filter: The cabin air filter, located behind the glove compartment or under the hood near the windshield, filters the air that enters the vehicle's interior. If the filter is clogged or damaged, it can cause airflow restrictions, leading to unusual noises. Check the cabin air filter and replace it if necessary.
Misaligned or damaged ductwork: The whistling noise could also be caused by misaligned or damaged ductwork. Check the ducts and connections in the AC system for any visible signs of damage or misalignment. If you notice any issues, it may require professional inspection and repair.
Ventilation system issue: There could be an issue with the ventilation system components such as the vents, grilles, or dampers. These components control the direction and flow of air in the cabin. A malfunctioning or partially closed vent or damper could create a whistling noise. Have a professional inspect the ventilation system to identify and resolve any issues.
It's worth noting that without a hands-on inspection, it can be challenging to pinpoint the exact cause of the whistling noise. If the issue persists or if you're uncertain about diagnosing and resolving the problem yourself, it is recommended to consult a qualified mechanic or automotive technician who can assess your car's AC system and provide appropriate solutions.