Sometimes the brake rotor heat shield can get bent and rub rotor. It needs a good 1/4" clearance as it can change shape and rub under load. Check around perimeter and pay special attention to the area near lower ball joint where clearance is tight. Remember, when you bent it on one place it moves elsewhere. So, it takes some patience to get it right. The brakes might also just be breaking in. Did they tell you to go easy on them for the first 50+ miles?
Then you progressively use them harder. Not doing so can make them make noises.
I am including a diagnostic walk through of other possibilities. Just follow the link below;

http://www.2carpros.com/articles/car-howling-noise

Answered by DrCranknWrench (expert)
3,292 answers provided

inspect the drum or rotor for anything scraping.the hub bearing maybe bad but they ussally whine thou.if you remove the tires and the brake caliper and rotor.than spin the hub and feel the bearing.this will give u the answer.

Answered by cadieman (expert)
2,858 answers provided