Cab is extra windy sounding

I've looked all over the vehicle and I just can't tell where the noise is coming from. If I'm on any kind of road going 40 miles an hour or more it just sounds really windy in the back of my truck like I have an window open. I don't have a rear window that opens and the back doors are both sealed up fine but I can't tell where the noise is coming from. We even had a recent heavy rainstorm and the back stayed completely dry so I don't think there's a major hole or anything but maybe it's just the back window seal? Has anyone dealt with a problem like this?

 

Check the air vents at the back of the cab in between the bed. They are on the back wall of the truck and there too relieve the pressure when you shut the doors. Maybe they’re stuck open. Unless you can fold your backseat down you may have to remove the seat to get to them. Also maybe it’s possible someone removed some sound deadening. Maybe under the backseat for speakers or under seat storage.