hi, iv got a 2001 zafira 1.8 comfort, for the last month it has been making a whistling noise when i accelerate, its like a turbo, if the car is parked up and revved it doesnt do it, if i put the car in neutral whilst driving and rev it the noise is there, took it to a mechanic and he doesnt know what it is. iv checked the throttle body/pipes ect and theres no leaks, can anyone please offer any advice on what it may be, thanks</font>