honking chatter under hood when put pressure on break pedal

When I am stopped, even when the car is powered off, I get a "honk-honk" noise and sometimes "whoo whoo" sometimes both, noise under what seems like the middle of the hood. I cant tell where, but I can replicate the noise anytime by just adding or releasing slight brake pressure. I think it is getting worse. It use to do it very seldomly, or only if I made it do it on purpose. Now it does it just about every time I come to a stop, or just as I take off.
Both dealerships say they have never heard of anything like this and that it must be my breaks squealing because they have reached the wear indicator. I had to tell each of them 3 times that it happened once the car was stopped or even powered off. They still say its break grind.
I have done enough break jobs, rebuilt enough engines, and changed out enough clutches to know what break wear indicators sound like, and to know they dont sound like rubbery honks coming from the center of the car, just as loud on the inside as the outside.
Some here must have experienced this before. Whats going on, and should I be worried, and why do the breaks have power when the car is turned off and there is no vacuum???

 

What was I thinking. Of course there is no vacuum when the motor is off. My 2007 has almost 70k on it. Thanks for the help.