Odd brake noise (not a squeal)
 

Just yesterday, my brakes started making an odd noise. The best way that I can describe it is a cross between a whistle and a whinning noise (perhaps an air leak somewhere???). I'm trying to figure out what this is and how serious it could be.

The noise seems to happen when braking at any speed although it's more noticable at lower speeds (probably just because it's easier to hear without the wind and engine noise). It's kind of hard to tell where it's coming from...sounds like it's coming from somewhere behind the driver's seat, though.

I know this isn't a whole lot to go on, but does it sound familiar to anyone? I can try to describe it more if that would be helpful. Thanks for any help / ideas you can offer...

-bryan