Aging steel roller bearings
Steel rollers ride harder, they're louder from the day they're installed, and by year 25 to 30 the bearings inside them are usually dry and pitted rather than just noisy. What we see on these calls: a door that grinds or hesitates a few inches into the track, one side lifting slightly out of square, or a roller that's actually cracked its stem where it meets the hinge.
