What would the cause of exhaust gas coming in to interior on idle but not while driving? Thanks.

 

When you are moving the drives away from the fumes. Think of this way the wind blows it away. When your sitting still is the only time your going to get fumes in the car.

Fumes in the car is from an exhaust leak. The exhaust system is suppose to cause the exhaust to exit from under the car. If you have a leak the hot exhaust gases just rise up into the car. They can enter through vents or rust holes.