Why would fumes come through my vents?

I have the same issue with my 2010 LS. Most likely, the issue is that u have an exhaust leak in your exhaust system. Your exhaust system is supposed to be a closed system from the engine to the tail pipe. If there is an exhaust manifold leak, then the fumes are filling the engine compartment and the "outside air" option will allow the fumes into the passenger compartment (thus, the smell). Furthermore, you wouldn't notice it as much while the vehicle is moving because the air passing under the vehicle is sucking the fumes FROM the engine compartment.

We just found and fixed leak in exhaust but it didnt help