Polaris Scrambler 95 400 cc HELP!

[WaRRioR350]

On my scrambler I have a button '' OVERDRIVE'' what does this do??? and when I start the ATV, I hear a weird engine noise like knocking... it never stops well when I give more throttle, I don't know if the knocking disapears but I just don't hear it anymore.. PS: I ONLY HEAR THE NOISE ON LOW RPM... I don't know, maybe it's bevcause off the noise 'cause I just don't hear it anymore...