I changed the air filter in my Fusion this month. Seemed easy, two clips hold it in place. It rained the next couple days, so I did not have my windows down while driving. The third day after, it was sunny, and I had the windows down to let in the air. That is when I heard a noise. It was similar to an exhaust leak, or a hole in the muffler. I was thinking I'd have to take the car in for service, as I could see no holes or rusted connections.
Then I remembered the air filter change. I popped the hood, and looked at it. Sure enough, it was clipped in, but the back side part was not in the grooves. I adjusted and re-clipped, and sure enough, the noise went away.I feel dumb, but lesson learned!