Factory defects. When they form the case,if any foreign tiny piece of debris,sand,whatever gets in there when they are manufacturing the case. Sometimes they will act up right away, and sometimes the tiny foreign matter will vibrate loose and dislodge itself over time showing up later. We used to see a lot of engines 5 years ago with that problem. It is less these days but it still happens ocassssionally.

Thanks,
Lance
Rcrenew.com