2004 Ford F 350 2wd Crew Cab Dually 6 0 Diesel Automatic Growling Noise That

I have a 2004 Ford F-350 2wd crew cab dually 6.0 diesel automatic. I have a growling noise that occurs at all speeds usually while going down hills with unaccelerated travel. I can make it make the noise by releasing the slightest amount of pressure on the trottle pedal and can make it go aways by either accelerating or relasing all trottle pressure. I have replaced the three u-joints and the carrier bearing, replaced the pinion seal and torgue the pinion yoke. The wheel bearing are tighten to spec and the front end is tight. I was wondering if you have any thoughts.
Thanks Bill