Engine loud going uphill under load

[Daryles]

2016 F350 7.6L Powerstroke turbo diesel dually.
We went from Banff to Jasper Alberta Canada on route 93. Along the way, north of Saskatchewn river crossing there's a place called big bend. After the bend the road climbs STEEP and LONG. I have no idea what the grade was, but it was STEEP!!
About 1/2 way, the engine started roaring. It was like a valve opened (or exhaust pipe was removed) and it got loud.
We pulled over at the next pull out.
As I let off the pedal the engine returned to normal. All instruments and temperatures were normal. We sat at the pull over for about 20 minutes then continued the climb. About 1/2 way up this part, same thing.
Again we pulled over at the next pull out.
Again as I let off the pedal the engine returned to normal.
Normally we dump our tanks before getting on the road however we have been dry camping in the NP here and the tanks were about 1/2 full. Maybe an extra 400 lbs?
We will definitely dump before going over that mountain on the way back.
My question is, is there some kind of bypass valve or overload valve under there that opens up under heavy load?
Already looked in Ford forums and our forum. Nothing.