I have a 2004 Pontiac Aztek and I have this noise coming from my rear brakes as well. I have 136, 000 miles on my vehicle. It occurred when I had new brakes and rotors installed. I took the vehicle back and they still made that cracking sound. The more they take the brakes off the louder it gets. I then took the car to two separate places and they are dumbfounded as to what is causing this problem. It is embarrassing and annoying. I was told to try and buy more expensive parts but that is not guaranteed to stop this problem. If there are any other ideas on solving this problem, I would love to hear them.