Same on my FZS600, i bought it and it was rattling quite loud at 15,000 miles, and over the next 15,000 miles it got steadily louder and louder, then at about 35,000 miles it just stopped one morning when i started up, and it has been quiet ever since, bike is now on about 43k and purrs like a kitten, its running better than it ever has and i think these bikes just get better when they are a bit older with decent amount of miles on them