Well now if you checked everything and it speced out, where does that leave you? A spun rod bearing. Lost it's crush.. lack of oil, aerated oil, or over revved, sometimes you get that one [censored] bearing too, detonation (both audible & silent), heck, even a backfire. Lots of things can cause a bearing to spin. I seen Callies cranks do the same. Usually do to the above mentioned.