Still not solved this problem yet. Nobody is sure where the source of the leak is although a leak is acknowledged by 3 different techs. Have been told the trial and error search may cost $$$$ of labour time. This is the only way. Has anyone got any other better search ideas, faster and more definite apart from the easy, replace your expensive vacuum box answer? Its a nagging buzz everytime you start off in the car, but lasts only about 5 to 10 minutes. For the time being, I put on the CD player at a decent volume to cut out this noise!!!