Rear brake knocking

[strudel]

If I use the rear brake firmly or drag it at low speed I can feel what is best described as a knocking sensation from the back of the bike (no abs). It's going back to the dealer soon for first service (only 88 miles), but I was wondering what the possible cause could be and whether it's expected or not. I'm thinking a warped rotor maybe? I'm hoping it isn't serious.