Well, if the bike routed a bare cable inside the top tube, then breaking some [hypothetical] cable guide in there could have easily introduced cable slack. That was my first thought. However, this bike seems to route continuous housing through the tube: if I pull the housing out at the entrance hole (front) it pulls in at the exit hole (rear) and vice versa. So the rear brake cable housing seems to be continuous. Under these circumstance it is certainly not clear where these 6mm of slack could come from (aside from pinch bold slippage, as you noted, or something snapping into place in the brake lever mechanism).
At the same time, I'm fairly certain that it wasn't caused by the slippage at the pinch bolt. Firstly, I recently replaced my brakes (installed Avid BB7) and measured the distance from the pinch bolt to the cable end. It was about 20mm. That length didn't change after the event. Losing 6mm from 20mm would immediately be noticeable. Now, after taking up the slack, I have about 25mm of the cable sticking past the pinch bolt. Secondly, when I installed the new brakes I put some light grease on the cable as it passed through the cable guide arm on BB7 (per SRAM BB7 installation instructions). When I retensioned the cable, about 6mm of the greased portion of the cable pulled out of the BB7 rubber boot, indicating that I'm really taking up some slack that developed somewhere along the frame, not at the pinch bolt.
I tried to wiggle the cable housing at the point where it enters the top tube (front), and I hear a distinctive sound of some part moving inside and hitting the tube from inside. I'm pretty sure that's the source of the rattle I heard on my ride this morning. I can't see the part, but judging by the sound it is relatively large (could be an illusion, of course). I wonder what it is, how I can take a look at it.
Last edited by AndreyT; 02-22-12 at 01:12 PM.