Active regen...that's what it sound like to me. When idleing the truck uses the EB and VGT to load the motor to create heat in order to allow for regen of the dpf.

 

I have the same problem, It all started when i heard A high pitch whistle when i let off the pedal at around 45 MPH, i set up a appt at the dealership, they were about one week out. By the time i got it in there it was on almost all the time. I was told that the connections were loose and that it would take about one hour to fix. Well an hour went by when they pulled back onto the lot after the test drive. I met the mechanic and seemed surprised. He told me that he tightened everything going to and leaving the turbo and it still makes the sound. I had to leave at that point and i was told to come back later to get it repaired. as of today i havent been back to get it looked at and i noticed today on a very steep grade i turned on my EB and nothing happened. I turned on the TOW HAUL and still no EB. I will be calling them first thing in the morning to get this figured out...

 

mine will bark the turbo when I accelerate (light or heavy) after a stop before the engine is up to operating temps. i only noticed it after i installed my afe.
Exhaust brake engages its self to help the engine warm up it colder temps weather you hit the switch or not. I would bet thats what your hearing.

mooseman

 

Recently my truck started the light whistle again. Only thought it had gone away. Finally after about an hour a day for a few day's hood up trying to pinpoint what was up, I found 2 bolts in the exhaust manifold, only tightened about a quarter turn but whistle is now gone. Before it became a little more pronounced with e brake on, after warming didn't really hear it any more. Hope maybe someone else might have the same luck as i did.