Well if you are actually locking the rear, I'd say it's the chain. Put the bike in gear, and with the clutch held, roll the bike forward a foot or two. Then quickly let out the clutch (as if you were to accelerate quickly). Does it make the same sound when you release the clutch? That's just the slack being pulled out of the chain. If it is, I'd just check that the chain slack is within spec, and if it is, then there is no problem. When my rear tire is on the limit of traction when braking, it starts to chatter sometimes. Also islandboy, if you feel a clunk at the front end after using the front brakes, (mine was specifically after releasing the brakes) I'd check the head bearings. I used to have a clunk too, until i tightened my bearings up a bit. No more clunk.