I had like a mildew smell a week ago coming out the vents. I drove with the AC on and the windows down for a few and it went away. I think it was something in the vents.

Most of the time the bad smells in my car are usually right after a loud unexplainable noise and my back window fogs up. Then, the smell creeps up on you just like a Ninja, Woe Nelly! It's enough to make an onion cry.