I have a 1998 Acura Integra with 204k miles on it. Some mornings after the car has been parked in my driveway facing up a slight slope overnight with the handbrake set, the first time I apply the brakes while backing out causes this very loud metal on metal noise that reverberates through the car. The brakes are fine thereafter and don't stick or make noise when braking while moving forward. The simple solution is to back into the driveway, but I'd like to know what's going on. Any clues?
How humid is it outside when this happens? It could just be surface rust getting scraped away. This usually isn't very loud, but kind of a low metal on metal slushing/grinding sound.
It may also be that your brakes are "resonating", which is a term I just invented to describe the somewhat loud, low reverberations that are caused by the brakes quickly grabbing and breaking free of the rotor at a high frequency, 60-150 Hz or so (is my guess). I am able to do this in a lot of automatic cars (at any time during my trip, however) by pressing the brake only slightly, holding it at just the right spot. This allows the car to move forward at 1/2 MPH or so and it has this effect.