ANYTHING hitting ANYTHING! It can also be engine noise. I am new to this too. I finished my new engine, thought everything was great till I started to tune it. 20-30 deg. retard before the boost would be up to 10#???? WHAT THE F^%$ is that. Talked to Red A and he said false knock. O.K. great. I bought D.S. (thank god) and it tells when the knock counts stop going up (in my case 2.25 sec after WOT). So Red made a chip with 3 sec knock retard delay and life is back to normal again.