I thought about that also but motor doesn't really shake/vibrate ...Motor Mount ????
Engine shifts under load, torques the driveline in a different way in reverse ??? or no ????
Not stupid but I've already covered that base as well. I've run Chevron Delo 400 for the last year ...and just recently switched to Mobil 1 Truck/SUV synthetic (aka Delvac) ...with no change in noise at all. My knock is a knock where when people ride with me they ask ..."what is that knock"?TopHog, I know this may sound stupid but, I had the exact same symptoms you are haveing after I swithced to synthetic oil. I ran the synthetic over 2000 miles and the hole time it was knocking. I just went back to regular oil "15w-40" and my knock has went away.