After mowing with my very quiet D series Lawnboy that I just got running, I pulled out my C21 Lawnboy and noticed it has a kind of a rattling knock sound to it. It runs great, has lots of power and the compression is way up there. Everything appears to be tight. Could it be piston slap? I figured the compression would be down if it was piston slap. What is required to fix it if that is that is the problem and what is most likely to happen if I leave it alone and run it for the rest of the season? TIA.