Weird noise while riding Yamaha YZF?

I have a 01 Yamaha YZF600R that I've had for about 5 months now. I got it from a friend, it was in perfect driving condition. Just last night I was riding and noticed a clanking noise and a jerking feeling whenever I down shifted and was slowing down in 1st gear. It only does it when Im in first gear and Im coming to a stop. Any ideas what this could be?