Hi, I took my recently bought Fx4 out to the lake for the first time. Had a ATV (700 lbs) in the box and towed a 14 aluminum boat on a trailer (1,200 lbs). I realize ur not really supposed to use O/D while towing, but I guessed because it was a lighter load it would be okay. While getting up to 65 MPH, I kept the O/D off, once up at speed for a minute, I would turn the O/D on. RPM's would be at 2,300 and drop to 1,600. This was on flat paved highway using cruise control and the truck behaved fine, no noise or tranny shifting up/down or higher temps. After an hour I started to get into hilly, tighter turning terrian. I set the cruise control at 40 MPH but forgot to turn O/D off. On the downward slope of a hill, the tranny operated normal, but as i went up the next hill, when the tranny went from 4th gear (O/D) to 3rd gear, it made a loud noise/vibration for a second or two and then went into 3rd. Almost sh*t my pants the fist time it happened. These were smaller hills/sharper hills (10 - 20 seconds to the top) with tight turn, sometimes at almost a dead stop while starting the climb. So I turn off cruise control experimented with turning off O/D. With it off, the noise/vibration never once happened, with O/D on it would happen almost every time.

I have never had this noise/vibration happen when out on the highway and accelerating hard up to speed with O/D on and NOT hauling/towing. Any thoughts what this would be?