thanks for your replies.
i know that it's a hydro boost brake. i also found an article in the internet about hydro boost brakes and they say that it's normal that you hear a certain noise when you push your brake pedal harder that normal.
when i push with normal force, there is no noise.
http://www.scribd.com/doc/6829161/Op...-of-HYDROBOOST
problem is that no matter where you search, when you find posts about this topic, there are always some who say that this is all normal and other ones who say that this should not be...