Never mind, I'm a moron. Or should I say the previous owner was. Looked at it again this morning with a clear head and something had to be hitting; it definitely sounded mechanical. One of the wiper arm pivot screws was about an 1/8" too long and it was preventing the linkage from making a complete cycle. I should have noticed that. I spent several hours trying to figure this out and rebuilt the entire linkage assembly needlessly. But oh well, I learned something and now have the smoothest wipers in town! Even though I will likely never use them, lol.