Whistling noises that occur when accelerating a vehicle are often due to vacuum leaks under the hood. If a vacuum leak is the cause, the car is likely to make the same noise upon pressing the accelerator when the vehicle is idling.
An automobile relies on constant vacuum pressure to operate correctly, according to 2CarPros.com. The engine uses a series of hoses and gaskets to maintain the vacuum. A leak allows air movement through a small space to set up high-pitched sound waves, creating the audible whistle. Increasing the throttle increases the vacuum and the speed at which the air passes through the leak.
Vacuum hoses attach to the intake manifold and supply the needed vacuum to accessories such as the power brakes. These small rubber hoses often degrade over time due to the heat of the engine. They tend to crack or break near the ends where they attach to other parts. The use of an automotive stethoscope is helpful in determining the location of the leak, as with other car noises.
As vacuum leaks grow larger they affect the performance of the engine. The leak often results in misfiring of a piston. If the misfire is significant enough, the vehicle's check engine light ignites.