I have avid bb7s on my recumbent and this problem just started out of nowhere. When I brake hard it sounds like there is a bad patch on my front rotor. (Later on at slow speed I found it sounded like a grinding noise rather then the usual squeal) but I would get some of it after I let off the brakes. Then it would be gone till I brakes hard again. Most of my rides are gentle braking so for the most part it is not there.
I took the wheel off and felt the rotor I could not find any bad spots. The rotor has a slight warp that has been there since I have had it. Itís less then the rear wheel one is. The brakes have about 1500 miles on them and they are on the second set of pads.
Well after I inspected them and could not find a problem I cleaned the rotor well and it was gone for a couple of weeks.
Well it came back if not a bit worse and it seemed to take longer for the sound to go away.
Now I could check the rotor out while the bike is stopped and it is rubbing the inside pad. But it does not always do it.
I have no clue about why it happens the qr is very tight as I have had the wheel shift a tiny bit with hard braking. The LBS had a spare rotor that some customer complained about the noise from so they gave it to me. It seemed to do the same thing but far less but it needs some use to really get grabby like it should be. But it stops after some seconds too.
I checked the bolts holding the rotor and the brake body.