I HAVE DISCOVERD THE CULPRIT! (or should I say,
culprits)
It's the two wall mounted vents on the rear of the cab. They are about 4" x 4" each. One is directly below the sliding back window and the other is next to it behind the passenger side rear seat.
Pulled all carpeting off the back wall last night and had the wife drive 39mph (most common speed that makes the booming noise) with all windows up while I was in the back with a flashlight. It's those damn vents.. The flaps are designed to only let air OUT of the cab. They are shut normally and never allow air to come IN the cabin. The vent flaps are super lightweight rubber (or maybe even paper) that 'flutter' with all the windows up at around 35-40mph. It's as if air wants to come in and go out at the same time. At slower speeds the vents flutter at a very fast rate and vibrate the back wall as well which is likely the 'booming' noise we're hearing. While they were 'fluttering' I stuck my fingers in the vents opening up the flaps and wouldn't you know it the fluttering stopped. This must be why as soon as you open a window the booming stops.
The problem is we need those vents to function as designed and let air out of the cabin for door closing, HVAC operation and airbag deployment, so to cover them up is DEFINITELY not an option. I thought about removing the flaps and applying some kind of foam filter instead, but not sure if that will introduce more road noise, humidity and dust into the cabin.
Anyone have a good idea how to stop the fluttering without either opening a window or removing the flaps and having the vents wide open all the time? The vents are removable. It's difficult to get them out but I believe you can force them out.