yeah could be the recirculation valve, for the sake of ?80 might be worth upgrading, i though this was one of the first mods to be done to any VAG turbo engine. Also might be worth checking all the hoses under the bay, especially intercooler and air intake hoses, may have perished rubber and letting air past under pressure, this would give the bigger lag and also the noise you describe. My money is on the recirculation valve though.