I was tightening my friends chain on her GS500 with the bike on the centerstand when I noticed the chain would tighten and then loosen as I spun the rear wheel. I made sure I had the alignment indicators accurate before tightening the wheel. At the chains tightest point it's enough to bring the tire to a stop while up on the stand, not enough to notice on a glide down the driveway but still a concern. I can't see any visual sign of warping on the sprocket or wheel. We decided to cancelled the ride. Any ideas?