I was out riding the other day on my hybrid (Trek 7100FX) and started coming down a steep hill. I applied my front and rear brakes progressivly as I was speeding up. This hill decends rapidly into a decreasing radius s-turn. I made it about half way down when I started hearing a loud clunking noise coming from my rear tire. It happens once per revolution no matter how light or hard I squeeze the rear brake. It dissappears when the brake is let go. There is no side to side movement with the wheel when it spins or is stopped. The wheel spins just as freely as it always has. The v-brake looks perfectly fine. It's not out of alignment and there is no noticeable wear on the rim.

What can be causing this? I want to fix it but I don't know what is wrong.
Thanks