Where is the BAC valve?

 

its located right on the uim. If you look in the fsm you can see a picture of it so it'll give you a feeling of what to look for.

 

Since your paint is noble white, I'm assuming you have an S4.

Since I own an N/A, this is information pertaining to that.

From the drivers side fender (American), look at the UIM. It should be attached to the side of it, near the top. It's a green cylinder. Probably the only green thing of that size under your hood. Hard to miss.