Why are my new brakes making a buzzing sound?

I had my car serviced at a Toyota dealer on May 5, 2017. They changed
the tires (from winter to four season), did an oil change, cleaned the back
(drum) brakes, and changed the front (disks and pads) brakes for new.
About a week later, I hear a buzzing sound coming from the front brakes.
This only happens when I come to a full stop (not when I slow down on a
highway) and only happens sporadically, not all of the time. The sound is
buzzing, like someone is rubbing against the treads of the tires to stop the
car. The sound only happens when i apply the brakes to stop and
disappears when i am stopped or let go of the brake pedal. Any ideas?