2008 F250 crew cab, long bed Lariat 6.4 diesel at 124K miles. Initial 90K miles under very light, highway conditions. I baby this thing. Some occasional towing of approximately 8k pounds prior to 2015 Since 2015, towing 12K 5th wheel. The first time I heard this was in November 2105, in the mountain area of NC @ elevation 2200 feet. Temp just above freezing. Pulling a long grade, maybe 4%. It sound like the old recoil starter on a Briggs and Stratton engine when they squeal. Or a dry bearing about to seize. I back off, and it quit immediately. I did not hear it again. Since then, most trips are not along any substantial grade, and the Turbo does not have to develop 30psi for any length of time. The 2nd time I heard it, the sound was identical, and the conditions were about the same. Long grade, higher elevation. Most recent, I drove the Blue Ridge Parkway, pulling the 12K 5th wheel, from Maggie Valley to Boone NC which is a 4 hour trip, climbing many significant grades. The noise was not as high pitched, and sounded more like a thin piece of metal revolving against cast iron, and not completely constant, but would quiet for a second then sound a few seconds then quite again. I noticed that it was only on long grades, where the turbo was having to produce around 30psi for a longer period of time. Maybe two minutes after starting up the grade. So I would back off from maybe 45 mph to 35 mph, to minimize the noise. I "THINK" it is from the Turbo, but I suppose it could be elsewhere. I have searched many forums and threads. I have found a few recordings that sound exactly the same as the first two times I heard it. Their sounds were recreated with the hood open, sitting still. One more thing I think MAY be important. I tow at 55 or less. I don't push it hard, except for the grades. When not towing, I don't push it or torque it. In fact it rarely goes into exhaust cleaning mode, except when towing, or soon after. The only thing I can determine is to continue driving until it gets worse, and to the point it has to be rebuilt or replaced. Also, engine is very strong. Serviced as required. I invite any discussion about this.