DIAGNOSIS This is a mechanical failure in the reciprocating mechanism. The primary cause is a worn or failed counterweight bearing or the drive bearing.
FIX The tool must be disassembled to access the gear case. Inspect the needle bearing (part of the counterweight assembly) and the drive bearing on the crank shaft. These bearings are prone to failure under heavy load. Replace any bearing with visible play, roughness, or damage.
IF THAT DIDN'T WORK The noise may be from a loose or damaged counterweight pin. With the gear case open, check that the pin securing the counterweight to the crank is tight and not sheared.
HEADS UP Running the tool with a failed bearing will cause catastrophic damage to the gear case and crank shaft. Stop using it immediately.