2005 polaris sportsman ho 500 fast clicking sound when in gear and driving

I just got a 2005 polaris sportsman and just barley put 100 miles on it, when I stopped for a second, then started going again when a loud, fast clicking noise started. I thought it was the tires, or something, but found out it's on the left side by my foot. I think it's the tranny. I tried putting it in neutral and pushed it fast to see if it would do the same clicking noise, and it doesn't. It does it only in gear. High, Low, Reverse. Any idea's what it might be?