was working on my bike (06 YZ450) and spun the rear wheel by hand while its up on the stand in neutral. I hear a knocking or ticking sound while doing this. I notice that it coincides with exactly one revolution of the drive sprocket and it sounds like its coming from that area. I checked to see if it was the chain or anything else exterior but could not find anything. The bike runs fine and I don't hear any abnormal noises while its running.....I changed the oil a week ago and everything was fine. I did the same thing on my son's yz125 just to make sure and it does not do it. Any ideas what this is?