DIAGNOSIS A new knocking noise is a critical mechanical symptom. The most common causes are severe carbon buildup (pre-ignition/detonation) or a failing internal component like a connecting rod bearing.
FIX Stop the engine immediately. First, verify you are using fresh, unleaded gasoline with an octane rating of 87 or higher. Old or low-octane fuel causes detonation. Second, check the oil level. Extremely low oil can cause bearing knock. If oil is low, refill to the correct level and listen carefully on restart.
IF THAT DIDN'T WORK The noise is likely internal. Do not continue to run the generator. The cause could be a loose flywheel, worn piston pin, or failed bearing. This requires disassembly by a qualified small engine technician.
HEADS UP Using gasoline with ethanol (E10) is acceptable but promotes carbon buildup if the generator is run frequently at low loads or stored without stabilizer. This buildup can eventually lead to knocking. Consider a decarbonizing additive for maintenance.