I have a 37 KAWI FX1000v motor. 300 hours on it. I was mowing today for about an hour and all of a sudden it started to knock heavily. I stopped and looked to see if maybe the oil plug came loose and it was dry, but nothing. Plenty of oil. I started it again and it knocked so bad it sounded like it wanted to bust through the side of the block. Almost sounds like something come apart and is in the cylinder hitting the piston. It runs fine and has power, just a BAD knock. Any ideas?