One thing that I had found years ago was the parking brake cable and bracket in the rear. If worked just enough on the road that it was a slight noise in the vehicle. If that is not it, get under the truck with some on in the drivers seat. Start wiggling and giggling brackets, cables, brake lines and exhaust system and you will probably find it.
Another place to check is a loose baffle in the muffler. This will usually show up on either acceleration or deceleration.
Amateur Radio Operator (KE5DFR)|Full-Time! - 2012 6.7L Ford Crew Cab Dually -2013 HitchHiker Champagne 38RLRSB - Travel with one Standard Schnauzer and one small Timneh African Gray Parrot