I bought an 08 90cc sportsman for my son knowing the starter, starter gear, and clutch gone out. I went ahead and ordered a new carb, starter, gears, plug, etc. It would crank with kick started, but ran little rough due to carb. I replaced everything last night (battery dead) and when I cranked it back up by kick starting it there was a loud roar noise and a lot of vibration I guess is the best way to say it, and it seems that the starter was turning while it ran due to the vibration I felt on the starter. I cut it off and took everything back apart, and made sure it was right and reassembled, and it was all correct. In doing that I lost the spring washer by the nut, so I just tightened the nut a little to see if it still made the noise when running. It started right up and did not do the sound then, but the kick start spring popped out so I had to cut it off and take it back apart. When I did this I found a washer and put it on, and tightened nut back down. Crank it back up same noise is back. Could this be an issue where I over tightened the nut putting things in a bind? When I lost the washer I didn't tighten it near as much as the other two times.
If it helps the noise sounds has a wind up toy car noise when pushing down on the kick start if that makes sense to you.