could be the squealer bar on the brake pad,
designed to rest on the rotor when the brake pad
wears down to a certain point.  you have about a
week to get new brake pads installed if this is
the case, the sooner the better.

in another case, once I had a small pebble lodge
itself between the back plate and the backside
of the rotor, caused squealing only when I went
certain speeds, as well as in reverse.  Of
course, I didn't know it was a pebble until I
pulled the whole bloody thing apart... lol

t