I guess it's possible that the axle is subjected to forces during braking that it doesn't get other times. Try using engine braking to slow the car and see if you get the same noise. It should load the axle in a similar fashion without using the brakes. Seems a little far fetched, but stranger things have happened. I've never had a disk get noisy from handling, and even if it did, I'd bet it would have done it from the beginning, not after 2500 miles.