Since a sliding patio door always rolls the same distance open and closed, the rollers in the bottom of the door make a complete revolution at the same place on the track consistently. Now, if over time, a ball bearing in a roller sticks at one point during the revolution, the roller will stop rotating for a split-second and the roller will “slide” along the track for up to an inch before it begins rolling again. Then, as it again revolves to the same rotation spot, it happens again and again and again.
Over the years, each time the roller slid, it wore down a little of the track creating a little valley every few inches, which can look like a roller coaster after a decade or two and make the door sound like it’s running over an unpaved road.
The most practical solution is the application of a stainless steel track cover after the rollers have been replaced.