take the wheel off and investigate how worn the pads are on the side that is making noise. It could be worn unevenly, and the metal warning scraper is just starting to touch the rotor, or alternatively you could just need to put some "disk brake quiet" lube on the backplate of the pad.
With brake noise, it really could be many things, but taking a look at the brakes is always a good idea and can get you moving in the right direction.