6 bolts holds that pulley on. Plus a big 1 1/4 inch bolt in the center. Someone may have done a timing chain job on it at one time and didnt get them seated well. You could even have a crack in the bottom pulley. If I were you, i'd be pulling the belts off, and checking the bottom pulley to see if it is loose, or the balancer is coming off the rubber, or the pulley having a crack or bent