A friend of mine had an ATC 250SX years ago and it did the same thing. It was the one-way bearing on the centrifugal clutch. The bearing was a little pricey though--about 50 bucks.

 

it is your starter, i had a 98 t-wolf that did the exact thing. Sometimes it would do it and sometimes it wouldn't. Your starter is a little loose. I had 3 bolts holding mine together and 2 of them were loose and one of them was missing. My starter sounded like it was breaking the teeth off when it was loose, does yours do this too? That has to be what your problem is, if not i don't know what to tell u to do. Hope that i helped!