OK, guys first post after all these years of reading and studying so take it easy on me if I don't press the right button.
I have a 07.5 6.7 3500 4x4 with the auto transmission. 30,000 miles. I've never had a bit of trouble and great mileage (until my last flash). I tow heavy but I'm not telling you how heavy . The problem is on my last trip towing, my transmission started throbbing in 5th gear. Just a low growl like a washing machine or out of round tire. Not a steady sound but a throb. Don't know how else to describe it. I don't hear or feel the sound in any other gear and it doesn't do it while not towing.
I just need to arm myself with info before talking to the dealer. Any ideas

 

I just had my 08 in for the first time for all the BS! Along with all the cleaning and replacing they did the latest reflash and now mine "throbs" in 3rd gear mostly. It feels like the converter is sticking. I have yet to tow my big trailer to see how it does.