I tow a fifth wheel weighing around 10,000 lbs and have a strange rumble/grinding noise from the truck brakes from time to time when the pads/rotors get hot on a long downgrade.

Most of the time, tow/haul mode eliminates the need for a lot of brake use, but from time to time, on very long, or steep, downgrades I have to use the brakes. At first everything is smooth, but after the brakes heat up, there is a grinding rumbling noise from the brakes, mostly from the front it seems. I've eliminated the trailer brakes as the source of the noise by using the hand operated brake control to operate only the trailer brakes. When I do this there is no noise.

There is never any noise from the brakes, no matter how much I brake, when the trailer is not on the back.

I've removed all the wheels on the truck and checked the pads and rotors. Everything looks perfect and the truck has only done 14,000 miles.

The noise almost sounds like a worn out pad does when the metal starts contacting the rotor. There is no vibration, only noise.

Any ideas?

mine seems to do this when backing the boat down the boat ramp. I haven't figured it out yet either

Rumbling noise im not sure, grinding noise gives me two different ideas.

1 the pads are metalic and the heat is exposing the metal faster
2. not brake related at all, wheel bearings under a load. Being pushed hard from the brakes and the extra pushing from the trailer causing the front end to transfer the load to the bearings.

Only possibilities, sometimes I find if you focus on 1 thing you overlook the others.