Describe the noise that first got your attention. Do you hear any new noise from the engine- knocking, rattling, tapping, clunking? Does any new noise change with light acceleration and deceleration- under road load, no load? Is there any misfire? Remove the spark plugs and look for oil on any of them. A spark plug wet with oil may indicate broken piston, but other possibilities also exist. A compression check is in order, as mentioned by hotshoe, but it may be appropriate to do dry and wet compression checks, and/or a cylinder leakage test.
Harvester
