Rusty flywheel. Drive the truck more often. That rust will go away.
We have a problem with disc brakes in the humid summers around here if you don't drive for a while.
When you start out for the day, it sounds like someone put a piece of 40 grit sandpaper on all of the pads, then when you hit the brakes it sounds like crush and run on the pads. After a while it self corrects.
