Yes, 65 mph seem to be the magic speed for this condition, seems to be affecting autos and sticks so we can rule out trans. Fred, it is interesting what you said about the exhaust. When I first complained of this to the dealer, that is the first thing STAR told them to check. They couldn't find any problem with it. Maybe at that certain rpm it is transmitting to much vibration through the exhaust hangers and setting up a harmonic imbalance? Maybe the hangers are too soft or too hard? I have noticed that the Cummins seems to be very "boomy" at around 2000 rpm at any speed (noticed accelerating through the gears), maybe at a constant 2000 rpm (65 mph with 4. 10 axle) the "boominess" is at just the right harmonic pitch that is is mixing with road and driveline noise and causing our problem? Not sure how they're going to come up with a fix for this one...