Strange Symptoms

My 2004 Mini Cooper recently began having 2 strange symptoms. When I am slowing down (at low speed) I often now suddenly get a power surge and when I then put the brakes on, I hear a loud, grating sound as if parts are loose. The sound is unlike any braking noise I have ever heard. Sometimes I hear the sound when I apply the brakes and it is not associated with the power surge. Can anyone explain this or recommend what to do?