It's the traction loading and unloading while on a medium to high grip surface. When tension builds up during a turn, and then one wheel looses grip there is a backlash in the drive parts and that makes the noise. The part tolerances on these machines are large. You might be able to make the noise lessen with brand new parts, but it will soon return with a little run-in. I have actually watched my machine spit out a little grave the same time the noise happens in a very slow turn. Go out on ice/snow or mud and make the same turn,,,, no noise. The only way to eliminate the noise would be to custom build parts with MUCH smaller tolerances.