Whenever I pass a car with my windows down, there's a whoosh noise. Why?

Im going to guess because your passing at such high speeds. For example if your driving 50 mph down the road and a car passes you going 50mph you pass each other at 100mph.
Just a guess though

I’m guessing it’s the sound waves your car is making, bouncing back off of the car you’re passing and reflecting at you. Like bat sonar, only bigger!

Part of it is air being pushed around by the chassis, and a lot of the noise is the tires’ tread trapping air between the rubber and the road, then pushing it back out. That’s mostly what you hear if you stand outside and listen to cars roll by from a distance (apart from engines).

You’re hearing the sound of your car reflected off the other car, as laureth and JoeyDesignsStuff said. This is actually a really interesting thing to listen too—you can hear objects on the side of the road (like rocks and signs and trees)—depending on how your car’s sound reflects off of them.

Our hearing is actually amazingly acute, but most people leave sound processing to their subconcious, it turns out ot be amazing what you hear when you stop and listen =)

it’s the air that is being displaced from the body of the car.

thus when you get something going fast enough like.. say a fighter jet and it breaks the sound barrier that means that it’s passing the speed of sound and gets in front of the waves, which causes a shock wave.