I've got a 2010 Ford Focus SES and a couple times lately (very random, very rare) while braking, I hear a noise coming from the front end that is sort of deep and like 'wuh wuh wuh wuh.' I thought of metal on metal, but nothing feels like it's grinding. The car itself is a Certified Pre-Owned, now up to 47,800 miles. Tires are fairly new with just over 5,000 miles on them. I just had the car checked out last week too and nothing negative was marked with the brakes. The fronts were listed at "70% 7MM" on my last check up. Is it something to be concerned about? Is it a brake issue or possibly something else?