There are few different ways to approach this sound. The first is the growl in the back of the throat. You can do this in any register of the horn. The second (and most common for altissimo) is a breaking of two notes. Play the fingering of the note you wish to be heard, add the growl in the back of the throat, and then alter your air speed (I feel you should slow the air down but there is no perfect way to do this). Essentially you are giving the perception of two notes being played but are allowing the one being fingered to be dominant. It sounds more complicated than it acually is. Listen to guys doing this on recordings and imitate thier sound. Hope this helps Tracy