Brake noise when pedal depressed

Ok, before any of you smart a** people respond with comments about my subject line, I'll beat you to the punch.

"Maybe some sort of anti-depressent brake fluid would help"
"My brake pedal was depressed until I stopped pushing it all the time"

C'mon, you can think of some more!

When at a stop light and foot on the brake, we noticed a while back that if the brake pedal is depressed a bit further, something makes a quick, audible, medium-pitched grunt. That's the best I can do on a description. It sounds as if it is in the engine compartment, but I haven't tried it with the hood open and my wife pressing the pedal. I need to do that.

I plan to take it by the dealership at some point but was wondering if any of you might know if this is normal and/or what it might be.