Yes. A knocking noise from the engine indicates a serious internal problem. Stop using the saw immediately.
- Possible cause 1: Pre-ignition (pinging). Caused by using fuel with too low of an octane rating. Always use high-octane gasoline in the 50:1 mix.
- Possible cause 2: Worn crankshaft bearings. Allows the piston to rock, creating a knock. This is a major repair.
- Possible cause 3: Loose or damaged components. Check that the muffler, clutch cover, and all external bolts are tight. A loose muffler can sound like a knock.
IF THAT DIDN'T WORK: The issue is internal. Continuing to run the saw will cause catastrophic engine failure.
HEADS UP: This is not a user-serviceable issue. Do not attempt to dismantle the engine. Transport the saw to a qualified service center for diagnosis and repair.