Why is there a knocking noise in your Harley Davidson?

Unless the motor has very high mileage and has been lugged regularly (causing connecting rod bearing failure), the most common cause of "knocking" in a Harley is a loose or worn compensator.

Drain the primary fluid, remove the outer primary cover and check torque value on the compensator nut. If it's within specs, remove, disassemble and inspect the compensator.

If the compensator's in good condition, check primary chain tension. A loose chain can slap the "ceiling" of the chaincase, causing a knocking noise. If that's the case, it will leave wear marks in the aluminum.