My Ford F250 PSD has a noise under load / pulling my 5th wheel. When getting on the ramp to the highway I accelerate and here a scraping, grinding, metal type noise from the driver side front. Let off the accelerator and it stops, push down and there it is. Only does it upon acceleration onto the on ramps. Scares the hell out of you that something is ready to break. Ask dealer about it and of course they could not find anything wrong. It did it again last weekend so I purposely kept on the accelator until it stopped. Nothing broke but still makes you think something is not right. Trucks on down the highway and up any grade with no problems. Anyone have any thoughts?

 

Kinda hard to tell when talking about front end noise like this. Maybe the front hub is engaging somewhat? A vacuum leak somewhere?

 

This is just something to look at.
When your towing and accelerating this is when your engine, trans, and driveshafts move the most. If you have a bad mount or bushing your drive shaft could be rubbing when under extream torque.
Could be way off but worth checking.