[OHPV-list] very weird disc brake issue

[steve]

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. 
It’s 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.

I fist replaced the rotor with a slightly used older model as the LBS 
gave it to me. though the brakes don't grab well yet it seems there is 
still some of it. I changed the pads today they are only about 1/2 worn 
and only look a bit rough.
but it would take some breaking in to really know but it seems like it 
is still there though right now it's only a tiny sound.
it's like the rotor bends for a bit on that one spot with hard braking 
then goes back to normal.