Lino, The idle screw has an o'ring on it that keeps it tight.The o'ring gets hard from age/heat. Change the screw (not expensive from VAG),or change the o'ring.I had the screw fall out on one of my cars. On the turbo cars when you open the throttle it really whistles, all the ladies turn around to look. File the last under usefull information. Smitty