DIAGNOSIS A rhythmic knocking is mechanical. The most likely cause is a loose mounting bolt for the engine or the alternator assembly.
FIX With the generator OFF and COOL, check all major mounting points. Use a socket set. Tighten the four engine mounting bolts. Tighten the bolts securing the inverter/alternator assembly to the engine block. Check the recoil starter housing bolts. Run the generator and listen for change.
IF THAT DIDN'T WORK The noise may be internal. Check the oil level immediately. Running with low oil can cause valve train noise and severe damage. Ensure oil is at the full mark on the dipstick. Use the recommended SAE 10W-30.
HEADS UP Do not ignore this noise. A persistent knock from the lower engine could indicate connecting rod bearing failure, often from chronic low oil. Continued operation will cause catastrophic engine seizure.