It sounds like a door is open or window is down on the rear passenger door of crew cab?

Many car owners notice this problem.

If any door or window is open,then do you see any Door ajar indication on dash or do you feel any door is not getting locked properly or window glass is loose.

If not then This can be one of the vents for the cab. There is one in each corner behind the seat.
Properly check to see if it is stuck open or has fallen out of place?

Getting this possibilities inspected will help.

Thanks.

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see below list of solved car problems :-----

Car takes time to Crank and start?

http://howtobyme.blogspot.com/2011/12/car-takes-time-to-crank-and-start.html

Car jerks while going uphill and Check engine light is ON?

http://schematicsdiagram.blogspot.com/2011/12/car-jerks-while-going-uphill-and-check.html

Car turns over, but only starts, if starter is jumped?

http://schematicsdiagram.blogspot.com/2011/12/car-turns-overbut-only-startsif-starter.html

Vehicle will NOT NAVIGATE FROM TPM SCREEN TO MAIN MENU SCREEN?

http://schematicsdiagram.blogspot.com/2011/12/vehicle-will-not-navigate-from-tpm.html

Service Engine Soon light stays on?

http://schematicsdiagram.blogspot.com/2011/12/service-engine-soon-light-stays-on.html

Gasoline around gas cap?

http://schematicsdiagram.blogspot.com/2011/12/gasoline-around-gas-cap.html

Cruise control not working?

http://schematicsdiagram.blogspot.com/2011/12/cruise-control-not-working.html

Car will not run after resetting inertia switch on Mercury Mystique?

http://schematicsdiagram.blogspot.com/2011/12/car-will-not-run-after-resetting.html