Took the truck to the dealer - they replaced the AVC module per 'Star Center' recommendation. I've driven the truck for 5 days since then - have 3400 total miles on it - no joy in Mudville - the noise is still there. Comes on within the same parameters as before. It seems to correspond with the 'ECO' light on the dash and I've read that post on here. Pushing the Tow/Haul button DOES cancel the noise. Will be taking it back to the dealer some time. Spending to much time at the dealership service department for a brand new truck.