Welcome to the club.
Take note of when the noise happens. If you've been parked uphill with the weight of the bike resting on the rear brake, the next time you use your rear brake to slow down it'll "klunk". That's a normal noise.
I did the same thing - took it in under warranty and had them "fix" it, only to have it reappear a few days later and be told not to worry. 40,000km later it's still there and hasn't gotten any worse.
