i have a big bear 350 4x4 and there is a loud knocking or tapping coming from the motor somewhere. it sounds more like it's coming from the head but i'm not sure. it also seems like i've lost some power in it. i have already changed the camshaft and the oil pump. i have also ajusted the valves numerous times. when i opened the oil pump there was some plastic or rubber in it, it looked like cylinder bearings but they were not metal. i don't know what it is or where it came from