i have a audi a4 1.9tdi 130pd 2001. over the last month or so i have noticed a weird sound possibly air coming back through the air box. when revving the car and revs start to drop seems like a type of dump valve noise i have read the codes on the car vagcom only 1 code brake light switch but thats it. i did find a post somewere cant remember were which showed a car making the same noise when revving this pointed to a diverter valve i thought i had sussed the problem but later found out that pd engines dont have them so back to square one.i have the car booked in at a garage next week but dont enjoy been ripped of off them any help will be much appreciated thanx