The noise you are hearing is a brake squealer which is attached to the brake pad. When the brake pad wears low enough that they need to be replaced this piece of metal rubs on the rotor and makes noise. Since it's only a thin piece of metal it vibrates when you're driving, but when you apply the brake pedal it stops the noise.
This brake squealer is only to let you know when your brakes are worn.
I would suggest that you have your brake pads inspected. Most likely it's time to replace them.
〉 Answered on Jul 5th, 2007 by Lori Johnson, Owner and Instructor at Ladies Start Your Engines