I read online that the small marks were caused by a worn drum seal. It seems thinner fabrics (like shirts, sheets, etc.) can get wedged into the gap between the dryer wall and the drum and get marks.
I bought a new seal from APP and referred to their video online. The repair process was exactly as it was shown in the video. I had to remove the dryer top and front, remove the drum, and take off the old seal. Then I scraped off the old adhesive and glued the new seal onto the drum. After letting the glue dry an hour, I re installed the drum. The only part that was a little difficult was balancing the drum in place to reattached the belt, but that was solved by enlisting a helper. The dryer works like new now.