transmission making noise

[postalman1845]

Hey new to this.Hope I am in the right forum.I have a 2000 ranger 4x4. It has the big v6 4.0.motor.I have been taking care of it for the last 7 years.Now driving on the interstate about 70mph I will hear a clash.Happens real quick.Doesnt hold back you reall dont feel it more just hear it.I am always in high gear with the od turned on.Happened to me 10 times last night on an hour drive.Just a clash and thats it.Kind of sounds like something wanting to mesh gears is best way to desribe it.I am in two wheel drive and the switch is not halfway its all the way in the two wheel mode.Doesnt seem to do it as much in slower city driving mostly what I drive is highway.Quick clash and its over.Cant feel anything just hear it.Changed fluid and filter 40000 miles ago.Fluid is clean no burn smell.Picked up flud and filter gonna change it again tommorow.Other than this it shifts fine all around.