2007 Mitsubishi Grandis Brakes

Good day! I noticed that every time I had to brake hard and depress the brake pedal suddenly and deeply, I hear a metallic grinding sound, after which a swooshing, whistling sound can be heard when I release the brake. The swooshing sound goes on but disappears later, especially after I pump the brake pedal up and down. It has been like this since I bought the car. I described the noise to the Mitsubishi service center but they did not address it and no one can explain to me what it is. I saw a post here that seems to describing the same thing in his Grandis (same year), but have yet to come across another model that describes what I just wrote here. If this is normal for this make, is there anyway to minimize that swooshing sound? I forgot to mention that I have to apply the brakes deeper after braking hard wen I have to make stops. It goes back to normal eventually though.