My partner has a Kona Sutra with Avid BB-7 disc brakes. The front brake rubs intermittently on the rotor. I just went to see if I could figure out what the problem was and right now the disc runs true and quiet. The disc is a Shimano CenterLock on an Alfine dynamo hub.

Usually when we notice the rubbing it is part way through a ride. Our usual route takes us down a relatively steep hill early in the ride. I'm wondering if the rotor could be warping from the heat of braking down the hill – though I'm not sure why it would stay warped for the next 20-30 minutes (for the rest of the ride).

Any suggestions for things to check or ways to isolate the problem?