Why do i hear a whistling sound while driving sometimes?

If it is coming from under your hood, it might be that you have a vacuum leak somewhere in the engine compartment. If it's inside the car at a speed over maybe 40 or higher, you could have a door or glass seal leaking. Finding whistling sounds in a car is all about being able to recreate the sound when someone is with you and that person can begin to pin point where the noise/leak is coming from. A whistle can sound like it's from one place to the driver, but a completely place to someone sitting in a different location in the car. If you narrow it down to something under the hood, that is another area that it could be coming from a number of different places. For the 16 years I worked in a custom repair shop up in Seattle we would get strange sounds and leaks in to fix all the time, and some of these could drive you crazy trying to find them; that is why it was fixed as a time and material job. It could take as short as 30 minutes and as long as 2 weeks; there is no ONE thing that will be a cause for a rattle, whistle or leak in a vehicle.

I used to get this on my old car, check the rubber seals around the outside of the windows for small gaps. The air passing over the gap can be causing the noise.