Originally Posted by olfordsnstone

This may have nothing to do with nothing, but I also have had that damned whistle sound while towing up steep grades in hot weather for quite some time. I recently had a problem with my truck (towing 12000 lb 5th wheel) up a grade on a hot day, when my long time whistle got really loud and the truck lost power. It shifted down and just didn't have much umph. My stock temperature gauge went up quite a bit, almost to the hot line. I took it to the dealership, while still under extended warranty and the tech said that my truck compensated for overheating due to oil cooler problem and that was why my truck lost power. Once in to the oil cooler replacement job, the tech found that the EGR cooler also needed replacement. I have had no problems since, except the whistle that is still there (though a bit quieter now and the tech says the whistle is the fan clutch). Oh yeah, I also did an EGR delete just after this to make sure I will not have the failed EGR/Oil Cooler problem (hopefully) ever again.

I hope this helps, Tim