F150 transmission noise

I recently bought a used 2005 Ford F150 Super Crew with 45,000 miles on it, I believe the previous owner may have used it to tow a camper as it is equiped with a towing package including electric brake and transmission cooler, when on the highway there is a whining or howling noise that comes and goes fairly frequently (lasts about five to twenty seconds and occurs sometimes every few minutes and other times only every ten or fifteen minutes), I narrowed it down to the transmission by pushing the button that turns off the overdrive mode and it instantly stops the noise. The transmission shifts extremely well and seems to perform just like a model citizen other than this noise (it isn't a noise that can be ignored as even my wife notices it and her solution to these types of things it normally to turn up the radio until the noise cannot be heard). Am I in serious trouble or is there a simple solution to this??