I own a 2013 ram 1500 with the same issue. I recently had the front L CV replaced as well as the axle. I notice this sound happens at any speed and gets worse the faster the vehicle is traveling. I was thinking it might be the carrier bearing on the driveshaft but I have not had a chance to diagnose the issue with 4wd engaged. Do you notice the sound evne after a recent diff fluid change? I had an inspection done and all was clear but this sound irritates me and was definitely not an issue prior to the axle replacement. Hope your issue is not serious as I hope my issue is not serious as well. Let me know how things turn out or if you get any updates. Also you can check out my post about a whirring sound from the front end.