I have a brand new, well seven month old RAM 3500 Longhorn with 3825km on the speedo and that EXACT problem, dealerships (2) can not fix it plus they never heard of the problem, I did some research and was told it is the same problem you described to Denton.
The dealerships have both called the DSM and TechLine and were told there is no fix at this time. No one has offered to reflash the ANC module...FCA has been no help at all, eight calls later and all I have is a case #82664785, all those people The last guy I spoke to said he would sign me up for the vehicle buy back program but would not guarantee it would help, I told him I wanted my truck fixed not bought back, then he told me he could not help me, he was such a dope. Anyway on Tuesday I am going to go to my Purchasing Dealer and tell them to hook up the mikes and reflash the module with whatever their latest software is and see if it makes any difference, this has been going on for seven months!! All I want is my truck fixed. My drone gets so loud you can't talk to the person sitting beside you!! If you disable the microphones in the cab the noise will go away, u do have a Harmon Kardon sound system in your vehicle right? If it is an Alpine then this is not your problem. Harmon Kardon has a noise cancelling system incorporated into the stereo that is supposed to make the interior absolutely quiet, if it works!