I would first check for a loose bolt on the front brakes. It's happened to the best of us, which is why I always use thread locker now. I've only had a bolt come loose and go pinking away under the car (amazingly I hiked back up the road and found it), but a buddy of mine had the whole caliper come off.

Another thing would be the guide rod mount, which is basically what braces the wheel while the brakes are being applied.