noise when pushing the brake pedal hard at a stop

i just noticed this last night. i have a 07 with 65k miles on it. my wife said it just started doing this. when i first turned on the car it made this noise. my wife says its like a quack a duck makes. i would call it a buzz groan combination. when the brake pedal is pushed hard to the floor it makes the noise some times. the noise comes on and off at about 3 times a second when its happening. it sounds similar to the buzzing that happens after you turn the car off but lounder and has the 3 times a second pulseing to it. does anybody know what is happening. i think ill try to get a recording of the sound and post it if knowbody knows what this could be.