While whistling noises in a car are due to several different problems, a vacuum leak is one common reason for this issue. Vacuum leaks also cause the car to run rough and stall. Worn, broken or cracked vacuum hoses are a common source of the leak, according to 2CarPros.com.
The source of the whistle is sometimes an overworked alternator. This electrical component keeps the battery charged and provides power for lights and accessories. High resistance between the alternator and battery increase its workload, causing a noise that varies with engine speed. To stop this noise, car owners should inspect electrical wires and connections. Any corroded connections should be cleaned, and worn wired should be replaced.
Worn fan-belt pulleys make a sound that increases with speed. Worn belts also create a whistling noise. Belts should be inspected for signs or wear and replaced if necessary. Pulleys should be tight with little horizontal movement. Belts should be adjusted to the factory-recommended tension.
Noises that only occur as the car is moving are sometimes due to worn window seals. As the vehicle moves down the road, it forces air through these small openings, resulting in the whistle. Windows that are not fully closed increase the chances of whistling. Any worn seals should be replaced, and windows should be tightly closed. If the noise continues, door seals should be inspected for any small holes that allow air to pass.