On my 2012 F350, at about 55 mph, I start to get a "slapping" sound behind my left ear that sounds like the seatbelt slapping as if the window were down. The only problem is the window is up. It goes away if I fold in the mirrors. I do not notice this sound on the passenger side.

Any ideas? The only modification I have made is to change the turn signal covers in the mirror from stock to the Recon smoked.